Step 1: Prepare the Cake Tins

• Preheat the oven: Set your oven to 180°C (160°C fan)/350°F/gas mark 4.
• Prepare the tins: Grease and line two 20cm round cake tins with baking parchment.

Step 2: Make the Cake Batter

• Cream the butter and sugar: In a large mixing bowl, beat the softened butter and caster sugar together until light and fluffy using an electric mixer.
• Add the eggs: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next.
• Sift in the dry ingredients: Sift the self-raising flour, cocoa powder, and baking powder into the mixture, and gently fold them in using a spatula.
• Add the milk: Stir in the milk to loosen the batter and ensure a smooth consistency.

Step 3: Bake the Sponge

• Divide the batter: Evenly divide the batter between the two prepared tins and smooth the tops.
• Bake: Place the tins in the preheated oven and bake for 20-25 minutes, or until the cakes are springy to the touch and a skewer inserted into the center comes out clean.
• Cool the cakes: Allow the cakes to cool in the tins for 5 minutes before turning them out onto a wire rack to cool completely.

Step 4: Make the Chocolate Icing

• Melt the chocolate: Gently melt the dark chocolate in a heatproof bowl set over a pan of simmering water, stirring until smooth. Let it cool slightly.
• Beat the butter and sugar: In a mixing bowl, beat the softened butter and icing sugar together until light and fluffy.
• Combine the chocolate and butter mixture: Slowly pour in the melted chocolate and continue to beat until smooth. Add the vanilla extract if using.

Step 5: Fill and Assemble the Cake

• Spread the icing: Once the cakes have completely cooled, spread a generous layer of chocolate icing over one of the cakes.
• Sandwich the cakes: Place the second cake on top and spread the remaining icing over the top and sides of the cake.