Method

• 1.
  For the banana cake, Preheat the oven to 160°C fan-forced. Grease a 15 cm round cake tin, at least 6 cm deep, and line the base and side with baking paper. In a large bowl, sift together the flour and bicarbonate of soda. Using a hand-held mixer or a stand mixer fitted with the whisk attachment, whisk together the mashed banana, sugar, vanilla extract, butter, eggs and salt on medium speed for 2 minutes, until the mixture is light and creamy. Add the sour cream and whisk until just combined. Fold through the flour in two batches, until all the ingredients are combined. Gently fold through the walnuts. Pour the mixture into the cake tin and bake for 40 minutes, or until a wooden skewer inserted into the centre comes out clean. Remove from the oven and allow to cool for 20 minutes, before turning out onto a baking rack to cool completely. Once cooled, use a cake leveller or a long, thin knife to carefully divide your cake into three even layers. Cover with plastic wrap and set aside until assembly.
• 2.
  For the buttercream, place the sugar and egg whites in a heatproof glass bowl. Set the bowl over a saucepan of gently simmering water, and whisk until the sugar has dissolved and the egg whites are slightly warm to the touch (at least 40°C). (You can omit this step entirely if you are using pasteurised egg whites, and instead, simply place the sugar and egg whites directly into the mixer.) Transfer the mixture into the bowl of a stand mixer fitted with the whisk attachment. Whisk on high speed until the mixture has formed stiff and glossy peaks, around 10–15 minutes. Switch to the paddle attachment. Add the butter in thirds, and beat on high speed after each addition until incorporated. Don’t be alarmed if the buttercream appears curdled – it will become light and fluffy again with continued beating for around 2–3 minutes (I absolutely promise!). Add the vanilla bean paste and beat until just combined. The buttercream is now ready to be used. Follow individual recipes for instructions on how to tint and flavour further. If you are making buttercream ahead of time and you find there are bubbles visible when you come to use it, you may need to mix it further before use. Using a stand mixer fitted with the paddle attachment, beat on low speed for 2–3 minutes to eliminate the air bubbles. Place a third of the buttercream into a medium bowl. Add the peanut butter to the remaining two-thirds and beat until just combined. Use food colouring to tint the smaller bowl of buttercream to the desired shade. Cover the bowls with plastic wrap and set aside at room temperature in a cool environment until needed.
• 3.
  For the chocolate spikes and spheres, source food grade moulds of your desired shape (available online or at all good cooking stores). Put the chocolate in a clean, dry, heatproof bowl over a saucepan of just-simmering water (the water should not touch the base of the bowl). Gently stir with a silicone spatula until melted, then tint with food colouring to the desired shade. To make the chocolate sphere, fill at least two of the semispheres in the mould completely with melted chocolate. Firmly tap the mould on your bench to release any air bubbles. Use the cake scraper to scrape across the top of the mould to remove any excess chocolate. Allow the chocolate to thicken slightly (around 5–10 minutes). Lay a sheet of baking paper on the bench. Turn the mould upside-down and let the chocolate run back out onto the baking paper. Tap the side of the mould to assist the process. Once the chocolate has finished dripping out, scrape again before turning the mould back up the right way. Check that you have a nice, smooth layer of chocolate on the semispheres. Turn the mould back upside-down and forcefully place it face down on a tray lined with baking paper to create a thicker rim around the inner edge of the semispheres. Set the semispheres by placing the mould in the refrigerator for at least 30 minutes. Once the semispheres have set, gently remove them from the mould. Place both halves of each sphere on a warm frying pan or warm oven tray for 1 second only – any longer and they will melt too far and lose their shape! Carefully press both halves together and hold until completely set. To make the chocolate spikes, reheat the remaining chocolate until melted. Use a piping bag or spoon to fill the mould cavities with melted chocolate. Tap lightly on the bench to release any air bubbles. Place in the refrigerator for at least 30 minutes, or until the chocolate has hardened. To remove the chocolate spikes from the mould, gently flex the mould and use clean hands to pop the chocolate spikes out of the cavities. To adhere the chocolate spikes onto the chocolate sphere, working one at a time, use a warm frying pan or oven tray to gently melt the bottom of each spike and then press onto the outside of the sphere. Allow each spike to set before applying the next. Repeat this process until you are happy with the number of spikes on your sphere (note: adding too many spikes may cause your chocolate decoration to sit too heavily on your cake). For chocolate ease, melting chocolate in the microwave can be a great time saver. Place the chocolate in a clean, dry, microwave-safe bowl and microwave at 50% power, stirring at 30-second intervals with a silicone spatula until melted. If you have warm hands, wearing gloves will help prevent the chocolate from melting, or you can run your hands under cold water before handling the chocolate.
• 4.
  To assemble, working on a cake turntable, secure the bottom layer of cake onto a cake board with a small dollop of buttercream and then gently twist in place. Use an offset spatula to spread a 5 mm (¼ inch) layer of peanut butter Swiss meringue buttercream right to the edge of the cake. Repeat the process with the second and third layers, leaving the final top layer uncovered. If the filling gets too soft, leave it to rest in the refrigerator for at least 30 minutes to firm up to a more workable consistency. Use an offset spatula to gently crumb-coat the cake with a thin layer of coloured buttercream. Chill the cake in the refrigerator for 10–20 minutes; this will make it much easier to pipe and smooth the buttercream onto the outside. Apply another layer of coloured buttercream to the chilled cake and use a cake scraper to smooth the side. Finally, use an offset spatula to smooth the top of the cake by gently pulling inwards from the outer edge of the cake into the centre, cleaning the excess buttercream off the spatula with each scrape. Do not refrigerate, as the outer buttercream needs to be slightly tacky for the chocolate spikes to embed themselves in place. Using the photos as a guide (or you may choose to create your own pattern), gently embed the chocolate spikes onto the outside of the cake. Once finished, place the cake into the refrigerator for at least 1 hour to set the chocolate spikes in place. Adorn the cake with the spiked chocolate sphere once the cake is ready for its big entrance!
• 5.
  For best storage, decorate the cake with the spiked chocolate sphere just before serving. This cake can be refrigerated for up to 4 days. You can bake the banana cake 2 days in advance. Tightly cover the baked cake with plastic wrap and store in the refrigerator. Chocolate decorations can be made at least 1 month in advance. Place the chocolate spheres and spikes in an airtight container in a cool, dark environment away from moisture.
• 6.
  To best store the buttercream, cover the bowl with plastic wrap and set aside in a cool, dry place until needed. You can refrigerate this buttercream for up to 10 days or freeze for up to 2 months. Thaw frozen buttercream overnight in the refrigerator, then bring to room temperature (gently reheat in the microwave in 20-second bursts if needed). Beat the buttercream on low speed until smooth before applying to your cake.