Preheat the oven to 190C/375F/Gas 5. Grease and line a 20cm/8in square cake tin.
Beat together the butter and sugar in a bowl until light and fluffy. Beat in the eggs, a little at a time, until well combined. (If the mixture looks like it is curdling add a spoonful of the flour.) Fold in the flour. Gradually add a few drops of blue food colouring (don’t add too much or the cake will look green.)
Spoon the cake batter into the cake tin and level using the back of a spoon. Bake for 35-40 minutes, or until golden-brown and a skewer inserted into the middle comes out clean.
Remove the cake from the oven, set aside to cool for five minutes, then turn out and cool, upside down, on a wire rack.
For the buttercream filling, beat the butter in a bowl until soft and smooth, slowly beat in the icing sugar. Continue to beat until fluffy.
To assemble the cake, slice the cake in half lengthways, place one sponge half on top of the other and trim off the edges, ensuring that both cakes are the same size.
Cut the cakes in half again lengthways to make four long rectangles.
Slice each rectangle diagonally along the length to make two triangular cakes
Spread each cut side of the triangle with buttercream and jam and sandwich together with the other triangle to make four rectangular sections. Wrap each rectangular section in clingfilm and chill for at least an hour.
Stack the rectangular sections into a square, so that the cross-section resembles a Union Jack (use an image of the Union Jack as a guide).
Assemble the bottom half of the cake first by spreading each of the long inside edges with buttercream and jam and sandwiching together. Repeat with the top layer and chill both cakes for one hour.
Finally, sandwich the two cakes together by spreading the top of the bottom layer and the bottom of the top layer with buttercream icing. Spread the bottom layer with jam and sandwich together. Chill the cake for one hour.
Meanwhile, roll the marzipan out on a surface dusted with icing sugar to a 5mm/¼in thickness, it should be large enough to cover the cake.
Warm a little strawberry jam and use this to brush the outside of the chilled cake.
Turn the cake upside down on the marzipan and brush the underside of the sponges with the jam.
Wrap the marzipan around the cake, pressing it gently onto the surface of the sponges, and press the edges together to make a firm join.
Turn back over with the seam underneath, trim a thin slice off each end and place on a serving plate. Take a slice from each end of the cake to neaten the edge and it should reveal a cake version of the Union Jack.