Preheat oven to 180C/ 350F and line or grease two 7" cake pans.

In a medium-sized bowl, cream the butter and sugar together until pale and fluffy. 

Add in the flour, cocoa powder, and milk. Fold everything in using an electric whisk or wooden spoon until smooth. 

Divide the batter between the two cake tins and bake on the middle shelf of the oven for 25 minutes. Make sure it's baked all the way through by piercing a skewer through the middle- it should come out clean.

Allow the cakes to cool for around 10 minutes, then run a knife around the edges to loosen them from the pans. Transfer the cakes to a cooling rack until completely cold.

Prepare your chocolate icing by mixing together the butter, powdered sugar, cocoa powder, and milk together until completely smooth. 

Spoon half of the icing onto the first cake and spread it all over using a butter knife or spatula.

Place the second cake on top and spread on the remainder of the icing. If you like, you can save a few tablespoons to pipe swirls around the edges for decoration.

Leave the icing to set for around 1 hour at room temperature (or in the fridge) before cutting into slices.