Preheat oven to 180/gas 4.
Grease and flour the sides of two-20 cm sandwich tins.
Line the bases of the tins with grease-proof paper.
Put chocolate and milk in a bowl sitting above a saucepan of gently simmering water and melt the chocolate. (The bottom of the bowl shall not touch the surface of the water.)
Set aside to cool a little.
Beat butter until
very soft then add the caster sugar.
Continue to beat until mixture is pale and fluffy.
Add in the eggs, one at a time, continue beating then fold in melted chocolate.
Sift in flour, baking powder, cocoa powder, and the bicarbonate of soda, then fold in gently in the mixture.
Divide the batter between the two tins and bake in the oven for about 25 minutes, or until the knife comes clean out of the center of the cake.
Remove the cakes, and allow to sit for at least five minutes before removing them out of their tins, then place them on a wire rack to cool.