Preheat oven to 160C/140C fan forced. Grease two 7cm-deep, 22cm (base) square cake pans. Line bases and sides with baking paper, extending paper 5cm above edges of pans on all sides.
Place butter, golden syrup, sugar and 1 cup cold water in a large saucepan over medium heat. Cook, stirring occasionally, for 4 to 5 minutes or until butter has melted. Remove from heat. Stir in bicarbonate of soda. Cool for 20 minutes.
Add egg to butter mixture. Whisk to combine. Sift flours, ginger and mixed spice over mixture. Whisk to combine. Pour 1⁄3 of mixture into 1 prepared pan. Pour remaining mixture into another pan. Bake thinner cake for 35 to 40 minutes and thicker cake for 50 to 55 minutes or until a skewer inserted into centre comes out clean. Cool in pans for 10 minutes. Turn cakes, top-side up, onto a wire rack. Cover with a clean tea towel. Set aside overnight to cool completely.
Make Buttercream icing: Using an electric mixer, beat butter for 4 to 5 minutes or until pale and fluffy. Gradually add icing sugar, milk and vanilla, beating constantly until combined. Tint icing with white colouring, if using.
Place the thicker cake on a chopping board. Using a serrated knife, cut cake in half crossways to form 2 rectangles. Place thinner cake on board. Cut diagonally into 4 triangles (discard 1 triangle). Place 1 rectangular piece on a flat surface. Spread base with 1/2 cup icing. Top with remaining rectangular piece, base down. Spread tops of 2 triangular pieces with 1/4 cup icing each. Place 1 on top of the other, icing-side up. Place remaining piece on top. Spread a little icing over the centre of a large serving plate. Stand rectangular cake on plate, cut-side down (use picture below left as a guide). Spread top with 1/4 cup icing. Place triangular cake on top to form the roof of the house. Refrigerate cake for 30 minutes or until icing is firm.
Spread remaining icing all over cake. Using main picture as a guide, attach marshmallows to the roof to form a chimney and chocolate melts to make tiles for the roof. Arrange chocolate buttons to form eaves. Trim 3 candy canes to fit. Place 1 piece of candy cane along the top of the roof and remaining 2 pieces underneath the eaves of the house, as shown. Trim remaining candy canes to form a heart. Position on front of cake to form doors. Press pieces of white chocolate around base of cake.
Cut 2 thin strips, about 4cm long, from sour strap. Using a little icing, secure pieces to mint, in the shape of a love heart. Press mint onto the centre of the roof. Cut 18 x 3mm-thin rounds from sour straws. Shape 1 round into a love heart and secure to centre of mint with a little icing. Arrange remaining rounds on cake for windows and a door handle. Decorate cake with silver cachous, then sprinkle red pearls across the top of roof. Just before serving, dust cake with icing sugar.