Method

• 1.
  Preheat oven to 180°C/160°C fan-forced. Grease base and sides of a 20cm square cake pan and line with baking paper.
• 2.
  Place the flour, cocoa, bicarb, baking powder and 1 tsp fine salt in a large bowl and whisk until well combined and there are no lumps.
• 3.
  Place the butter, oil, and caster and brown sugars in a stand mixer fitted with the paddle attachment, and mix until pale and fluffy. Add the vanilla and mix well before adding in the eggs, one at a time, mixing thoroughly between each addition.
• 4.
  Fold the dry ingredients through the butter mixture, being careful not to overmix (you don’t want to activate the gluten in the flour or your cake will be tough), followed by the sour cream and milk.
• 5.
  Finally, pour over the hot coffee and mix until smooth. The batter will be quite runny, which is exactly how it should be.
• 6.
  Pour batter into prepared pan and bake for 55-60 minutes, until a skewer inserted in the centre comes out clean (a kitchen thermometer inserted in the centre should read 96°C). Allow cake to cool in pan for 5-10 minutes before removing from pan and turning out onto a wire rack to cool completely.
• 7.
  For the espresso mascarpone whip, combine all ingredients in a large bowl and whisk until thickened. Using a hand whisk is better for this, because if you overmix the mascarpone, it can easily split.
• 8.
  To serve, pile the mascarpone over the top of the cooled cake and level surface with a spatula. Liberally dust with cocoa powder before serving.