Cream together the butter, sugar and eggs until pale and fluffy. Cream in the baking powder, salt and cocoa until smooth. Add 1/3 of the flour and stir in until just combined. Add half the milk and stir until just combined. Add another 1/3 of the flour and stir in until just combined. Add the rest of the milk and stir until just combined. Stir in the last 1/3 of the flour until just combined.
Divide the batter equally among the circular and square pans. Bake for 25-30 minutes until a skewer inserted into the center of the cake comes out clean.
Leave to cool for a few minutes before turning out onto cooling rack to cool completely (remove the parchment paper from the bottom of the cakes too).