Preheat the oven to 180°C/160°C Fan/Gas 4. Grease two 18cm sandwich tins and line them with baking parchment.
Using a food mixer or a hand-held electric whisk, beat the butter and sugar until light and creamy. Beat in the eggs one at a time, adding a little flour with each egg to prevent curdling.
Sift in the flour and cocoa powder and mix on a low speed until everything is well combined. Add the milk and beat the mixture until it is smooth.
Divide the mixture between the prepared tins and level the tops. Bake the cakes in the preheated oven for 20 minutes until they are well risen and a thin skewer inserted into them comes out clean. Leave to cool in the tins.
To make the chocolate frosting put all the ingredients into a bowl and beat until they are smooth with a hand-held electric whisk.