Preheat the oven to 190°C and grease a 20cm square cake tin.
Cream the butter and sugar until fluffy, then beat in eggs and fold in flour.
Tint the batter with blue food colouring and bake in the tin for 35-40 minutes.
Cool the cake for five minutes before turning onto a wire rack to cool completely.
Beat the filling butter and icing sugar together until smooth and fluffy.
Slice the cake in half lengthways, stack them, and trim edges to uniform size.
Divide the stack lengthways again to create four long rectangular prisms.
Slice each rectangle diagonally along the length to create eight triangular sections.
Apply buttercream and jam to the cut sides, rejoin into four rectangles, and chill for one hour.
Stack the rectangles to mimic the Union Jack pattern.
Join the bottom layers with buttercream and jam, repeating for the top, and chill for one hour.
Sandwich the top and bottom halves together with icing and jam, then chill for one hour.
Roll out marzipan to 5mm thickness on a surface dusted with icing sugar.
Brush the chilled cake exterior with warm strawberry jam.
Place the cake on the marzipan and brush the remaining side with jam.
Encase the cake in marzipan, pressing the edges to create a firm join.
Position the cake seam-side down and trim the ends to reveal the pattern.