3 1/2 ounces dark (70% cocoa solids) chocolate, broken into pieces
1 1/4 cups light brown sugar
1 1/3 cups all-purpose flour
1 teaspoon vanilla extract
3/4 cup cocoa powder
3 extra-large free-range eggs, lightly beaten
1 teaspoon baking powder
1/2 cup buttermilk
1 teaspoon baking soda
Pinch of salt
2/3 cup ground almonds
14 tablespoons unsalted butter, softened, plus extra for greasing
7 ounces dark (70% cocoa solids) chocolate, broken into pieces (for icing)
7 ounces milk chocolate, broken into pieces (for icing)
18 tablespoons unsalted butter, softened (for icing)
Confectioners' sugar, for dusting (optional, for icing)
Preheat the oven to 350 degrees Fahrenheit. Grease two 8-inch pans with sides about 1 1/2 inches deep and line each with parchment paper.
Melt the chocolate in a heatproof bowl over a pan of barely simmering water, making sure the bowl doesn't touch the water, then set aside to cool.
In a large bowl, sift the flour, cocoa, baking powder, baking soda, and a pinch of salt, then stir in the ground almonds. Using an electric stand or hand-held mixer, cream together the butter and sugar until very light and fluffy.
Add the vanilla extract to the eggs. With the beaters running, very slowly add the egg mixture to the butter and sugar, adding 1 tablespoon of the flour mixture during the process to prevent curdling, then add the melted chocolate and buttermilk.
Very gently fold in the remaining flour and divide the mixture between the pans.
Bake on the center shelf for 30 to 35 minutes, or until risen and firm to the touch. Leave the cakes to cool slightly in the pan before turning on to cooling racks.
Once cold, remove the papers and slice each cake in half horizontally through the middle.
To make the icing, melt the chocolate following the method above. Set aside to cool slightly, then beat together with the butter. Using an offset spatula, spread evenly to sandwich the layers together and cover the top of the cake.