Preheat the oven to 170C/325F/Gas Mark 3.
Combine butter, caster sugar, eggs, self-raising flour, ground almonds and baking powder in a large mixing bowl and mix until all the ingredients are evenly incorporated. Divide the mixture into two bowls.
Add the vanilla extract and 1 ½ teaspoons of milk in one bowl and mix until fully incorporated.
Dissolve the coffee powder with 1 ½ teaspoon of milk, then add coffee liquid and chopped walnuts to the other bowl. Mix well.
Grease and line 20cm (8 inch) square tin. Pour the vanilla mixture in one half and the coffee and walnut mixture in the other.
Bake in the oven for 30-45 minutes. Check with a skewer to see when the cake is done. While waiting for the cake to cool, make the buttercream icing.
Dissolve the coffee powder with 1 ½ teaspoons of milk. Combine soften butter, icing sugar and coffee liquid in a large mixing bowl and mix until smooth and spreadable.
Cut each half of the cake into two equal strips. Lay one vanilla and one coffee strip next to each other and stick them together with the buttercream. SSpread more of the buttercream on top. Stick the other two strips together and place on top to create a checkerboard effect. Use remaining buttercream to spread over the cake.
Roll out the marzipan on a clean work surface dusted with icing sugar, until wide enough to wrap around the cake.
Roll the cake over the marzipan neatly and press the corners together to seal it. The join should be underneath. Options to decorate: crimping the edges by pinching marzipan; score the top of the cake by creating diagonal lines with a sharp knife; sift over with icing sugar; decorate with remaining walnuts.