Preheat oven to 180 degrees C. Grease and flour two 23cm square cake tins.
Cream together the caster sugar and softened butter until light and fluffy. Beat in the eggs one at a time then add the vanilla.
In a separate bowl sift together the flour, baking powder, salt and cocoa powder. Add to the butter mixture, alternating with the milk and stirring well after each addition. Divide the batter between the cake tins and smooth down.
Bake 30-40 minutes or until cakes spring back to the touch. Remove and cool in the tins for 10 minutes, then invert onto racks and let cool completely.
Decorate the cake:
Make sure the tops of each cake are flat, trimming if necessary. Sandwich the cakes together with buttercream icing, then spread icing evenly over top and sides of cake.
Take a golf-ball sized piece of the marzipan and knead a few drops red food colouring until it is evenly coloured. Set aside.
Roll out the remaining marzipan on a work surface you have dusted with icing sugar. Cut out a square section the size of the cake top - you can use a cake tin to measure this. Lay on top of the cake.
Re-roll the marzipan trimmings and cut into four strips measuring 3cm x 23xm, for the sides of the cake. Cut three square notches out of each strip. Carefully place a strip along the top of each side of the cake, notches downwards. Seal the marzipan edges where they touch.
Roll out the red marzipan and cut out some small squares of various sizes. Place in random Minecraft pattern on top of the cake, using dabs of chocolate buttercream as glue.