1. For the chocolate frosting, pour the cream into a saucepan, add the butter and heat, stirring occasionally, until the butter has melted. Bring to just below boiling point, then remove from the heat. Add the chocolate and whisk until smooth and glossy. Pour into a bowl and leave to set at room temperature, whisking occasionally.
2. Heat the oven to 180°C/fan 160°C/gas mark 4 and grease and line three 20cm (8in) loose-bottomed sandwich tins with baking parchment.
3. For the sponges, put the cocoa powder, light brown sugar, vanilla paste and 375ml boiling water in a bowl and whisk together until the sugar has dissolved. Set aside. Sieve the flour, baking powder and bicarbonate of soda together into another bowl.
4. Cream the butter and caster sugar together in a separate bowl until pale and fluffy, then beat in the eggs, one at a time, mixing in a tablespoon of the flour mix after each egg. Add the rest of the flour, a third at a time, folding well to disperse any flour pockets.
5. Fold in the cooled cocoa mixture, then divide between the three tins and bake for 25–30 minutes, until risen and a skewer inserted into the centre comes out clean.
6. Remove from the oven, leave to cool in the tins for 5 minutes, then turn out onto wire racks to cool.
7. For the decoration, melt the chocolate in a bowl set over a pan of gently simmering water. Meanwhile, grease the underside of a baking tray with oil and pour the melted chocolate onto it. Leave to set, then drag a cheese plane over the surface to create curls. Keep these cool.
8. Place a cooled sponge on a cake stand and spread with about a quarter of the frosting. Place another sponge on top and spread with another quarter of the frosting. Place the remaining sponge on top, then spread the remaining frosting over the top and sides of the cake, swirling with a palette knife. Arrange the chocolate curls on top of the cake.