Method

• 1.
  For the hazelnut syrup, place all ingredients and 1 cup (250ml) water in a small saucepan over medium-high heat. Stir until sugar dissolves, then boil, without stirring, for 3 minutes or until slightly reduced and syrupy. Set aside to cool to room temperature.
• 2.
  Grease base and sides of 4 x 20cm springform pans and line with baking paper. Divide gelato evenly among two prepared pans and level surface. Cover surface directly with plastic wrap, then enclose entire pan. Freeze until required.
• 3.
  For the chocolate cake, preheat oven to 180°C/160°C fan-forced. Sift flour, cocoa and a pinch of salt flakes into a bowl.
• 4.
  In a stand mixer fitted with the whisk attachment, whisk egg whites on medium speed until foamy. Add cream of tartar and whisk until well combined, then slowly add 1/4 cup (55g) sugar down side of bowl while whisking constantly. Increase speed to medium-high and whisk until soft peaks form. Transfer to a large bowl.
• 5.
  Using same stand mixer bowl (no need to clean in between), add yolks and extra 1/4 cup (55g) sugar and whisk on medium-high speed for 3 minutes, scraping down sides of bowl occasionally, until very thick and pale. Add vanilla and 2 tbs chilled water and whisk until combined. Add flour mixture, in two batches, and whisk until combined (it will be sticky and thick).
• 6.
  Remove bowl from mixer. Add one-third of the egg white mixture and fold in vigorously until well combined. Gently fold in remaining egg white mixture until just combined (do not overmix). Divide evenly among remaining prepared pans and level surface. Bake for 20-25 minutes until a skewer inserted in centre comes out clean. Cool in pans for 5 minutes, then transfer to a wire rack to cool completely. If necessary, trim tops of cakes to create a level surface.
• 7.
  Brush tops of cakes liberally with hazelnut syrup. Return one cake to its lined cake pan, syrup-side up, and set aside. Remove base of remaining cake pan and re-line sides with baking paper (you’ll need this later to assemble the cake).
• 8.
  Meanwhile, for the chocolate custard, using a balloon whisk, whisk yolks and sugar in a large bowl until thick and pale. Add cornflour, whisk until well combined. Heat milk and cocoa in a medium saucepan over medium heat, whisking, until it just comes to the boil. Slowly whisk into the egg yolk mixture until well combined. Transfer to a clean large saucepan over medium heat and cook, whisking constantly, until mixture starts to thicken. Immediately remove from heat and continue whisking for 30 seconds to make sure no lumps form (mixture will be thick). Transfer to a large heatproof bowl, cover surface directly with plastic wrap and chill for 20 minutes to cool. Whisk in cream until well combined and smooth.
• 9.
  Remove gelato cake pans from freezer. Stand at room temperature for 5 minutes to soften very slightly, then release from pans. Discard baking paper. Working quickly, spoon half of the chocolate custard over the chocolate cake in pan, spreading evenly. Top with a round of gelato, lightly pressing down so that the layers connect well and there are no gaps. Stack the second lined cake ring directly on top of the first pan to form a high edge. Place remaining chocolate cake on top of the gelato, syrup-side up, and gently press down so the layers connect well and there are no gaps.
• 10.
  Spoon remaining chocolate custard over chocolate cake, spreading evenly to edges and to level top. Place remaining round of pistachio gelato directly on top, lightly pressing down so that the layers connect well and there are no gaps.
• 11.
  Sprinkle top with hazelnuts and pistachios, lightly pressing into surface. Cover surface directly with plastic wrap, then enclose entire stack in plastic wrap. Freeze for at least 4 hours, or overnight if time permits. Remove cake from freezer and stand at room temperature for 5 minutes, then release sides of pans and discard baking paper. Transfer to a serving plate. Drizzle with any remaining hazelnut syrup to serve. Use a large knife dipped in warm water between each cut to make slicing the cake easier.