For the cake
1. Line the base of two deep round 20-centimetre sponge tins (not the removable base kind as the cake batter is quite liquid) with non-stick paper, and heat the oven to 180C (160C fan forced).
2. Put the sugar in a bowl and whisk in the milk. Whisk in the eggs then the oil and melted chocolate.
3. Sift in the flour, cocoa, baking powder and bicarbonate, whisk until smooth then beat in the coffee. The mixture will be very liquid, thick soup consistency, but don’t worry. Pour it evenly into the tins and bake for about 45 minutes or until a skewer poked in the centre comes out clean.
For the buttercream
1. Beat the butter with the icing sugar until smooth. Add the mascarpone and beat until just combined, no more. Then add the melted white chocolate and beat again until just combined and fluffy.
2. Spread half of this over the top of one cake, sit the other cake on it, then finish with the remaining icing though you will have a some left over. Stores well covered in the fridge, bring to room temperature and gently beat again until smooth to use.
Tips: The cake can be made dairy-free easily, by replacing the milk with almond milk or juice, and decorating with a simple chocolate icing.
The buttercream is utterly extravagant, and perfect for a bittersweet chocolate cake. The trick to keeping the texture light is to avoid overbeating it. Makes more than you need to cover and sandwich a 20-centimetre round cake thickly.
Dining and Cooking