For the chocolate fudge frosting, pour the cream into a pan, add the butter and heat, stirring occasionally, until the butter has melted. Bring to just below boiling and 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.
Heat the oven to 180°C/160°C fan/Gas 4 and grease and line the base of three 20cm loose-bottom sandwich tins with baking parchment.
Put the cocoa powder, light brown sugar, vanilla paste and boiling water in a bowl and whisk together until the sugar has dissolved. Set aside to cool slightly.
Cream the butter and sugar together in a separate bowl until pale and fluffy, then add the eggs, one at a time, mixing between each addition.
Sieve the flour, baking powder and bicarbonate of soda together in a bowl.
Add the flour mixture to the egg mixture in three separate batches, beating well between each addition.
Fold in the chocolate mixture, then divide the mixture between the three lined tins and bake for 25–30 minutes until risen and a skewer inserted into the centre comes out clean.
Remove from the oven, leave to cool in the tins for 5 minutes, then turn out onto wire racks to cool.
For the decoration, melt the chocolate in a bowl set over a pan of gently simmering water. As soon as the chocolate is melted remove the bowl from the heat.
Lay a sheet of acetate on a cold marble slab and pour the melted chocolate onto the acetate. Using a palette knife, spread the chocolate backwards and forwards over the acetate until it starts to change colour and is beginning to set. Using a knife, score various triangular shapes.
Using the tip of the knife, lift the corner of the acetate sheet off the marble slab and transfer the acetate sheet onto a baking sheet. Place the baking sheet in the freezer until the chocolate has set hard.
Place one of the cooled sponges on a cake stand and spread with about one 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.
Remove the chocolate from the freezer and peel the acetate away from the chocolate shapes, revealing the triangles. Arrange the chocolate triangles on top of the cake and spray with edible gold spray.