Preheat the oven to 180ºC/gas mark 4. Grease and line a 22cm x 32cm rectangular cake tin and a 2lb loaf tin. In a bowl, stir together the cocoa, 125ml boiling water and sunflower oil. Allow to cool slightly.
In a large mixing bowl, use an electric handwhisk to beat the melted butter, sugar and vanilla essence until pale and smooth. Beat in 1 of the eggs and 1 tbsp flour until smooth, then repeat with the remaining eggs. Beat in the cocoa and oil mixture, then fold in the remaining flour and baking powder.
Spoon ¼ of the mixture into the loaf tin and the remaining mixture into the other tin. Bake for 50 minutes, then take the loaf cake out of the oven. Let it cool in the tin for 10 minutes, then turn it out and leave to cool completely on a wire rack. Leave the larger cake to cook for a further 25 minutes until risen and the cake springs back when lightly pressed. When ready, place on a wire rack. After 10 minutes, turn it out of the tin and leave to cool completely.
Meanwhile, make the buttercream. Beat the butter with an electric hand whisk until light and fluffy. Gradually add the icing sugar and beat until smooth.
Trim the 22cm x 32cm cake so the sides are straight. Place on a work-surface and cut two 11cm-wide pieces of cake from one end. From the remaining strip of cake, cut two 5cm-long pieces – one for the chimney and the other for the porch. You will have a small piece of cake remaining. Cut a triangular wedge from the bottom of the chimney piece. This will be the roof of the porch. Sandwich the two 11cm-wide pieces together lengthways with some of the buttercream (one on top of other) to make the main part of the house.
Using a serrated knife, level the surface of the loaf cake, then cut it diagonally across lengthways. Flip the pieces over so the straight sides are touching, then sandwich the pieces together with some buttercream to make the roof. Put the house part of the cake on a large cake board and coat the top and sides with most of the buttercream. Coat the porch piece with the remaining buttercream and set aside.
Knead together the yellow and white icing. Dust a surface with icing sugar and roll the icing out to a 4mm thickness so you have a rectangular shape big enough to cover the house part of the cake. Use the rolling pin to drape the icing over the cake, then smooth down to make an even surface. Trim away any excess. Reroll the trimmings to a 3mm thickness and use to cover the porch. Brush one side of the porch with a little water and press against the front of the house to attach, then add a piece of mint stick as the doorstep.
To make the chocolate ganache, melt the chocolate in a bowl set over a pan of simmering water. Remove from the heat and stir in the double cream, then transfer to the freezer for 20 minutes to cool and thicken. Spread most of the chocolate ganache over the roof and dot some on top of the main part of the house. You will need to leave some aside for the chimney and the porch roof. Carefully lift the roof using two long palette knives and position it on the house.
Gently press the crunchy mint sticks on to each end of the roof, snapping them to size as you go (use the offcuts for the corners). Put the chimney in place with the triangle cut side-facing downwards, so it fits snugly on the roof. Coat it with some of the remaining ganache.
Press on a row of chocolate mint thins along the bottom of one side of the roof, then continue to the top, overlapping each row as you go. Repeat on the other side. Use a small palette knife to smooth the ganache on the chimney and add some detail.
To make the porch roof, spread the remaining ganache over the triangular piece of cake, then decorate with small pieces of crunchy mint sticks and two chocolate mint thins. Stick it into position on top of the porch.
Gently knead together 25g each of the black, green and red icing until the colours are marbled. Dust a surface with icing sugar and roll out to 3mm thickness, then cut four window shapes, each one about 5cmx4cm. Roll out 25g red icing to a 3mm thickness and cut out a 5cm x 4cm door. Using the back of a knife, lightly score on some lines to make door panels, then roll a small piece of black icing into a ball and press on to the door for the doorknob. Attach the door and windows to the cake using a little water.
Put ¼ of the royal icing into a piping bag fitted with a small round nozzle. Pipe on window frame details and icicles around the top of the chimney and the porch. Use small dots of the royal icing to attach some of the snowflake sparkles around the edge of the roof and the chimney, the holly decorations above the porch and on the corners of the house, and some of the chocolate beans around the door.
Use the remaining red and black icing to roll and mould Santa’s legs and boots, joining them together with a little of the royal icing. Put the legs in place on top of the chimney. Use the remaining green icing, the silver lustre, some of the chocolate beans and snowflake sparkles to make the Christmas trees.
Use the remaining royal icing to coat the cake board, making peaks with the flat side of a knife. Add the snowman and trees and finish with a dusting of icing sugar snow.