The pistachio mixture in this recipe makes a great base for a stunning cake (made to look wintry, as here, or like your own home, if you prefer). Try to find Iranian pistachios, as they give such a great colour.
Heat the oven to 180°C/160°C fan/Gas 4. Make the sponge. Beat the butter and sugar in the bowl of a stand mixer fitted with the beater attachment until light and creamy. Gradually add the egg, beating well until smooth, and adding a little flour if the mixture curdles.
Add the flour, baking powder, and ground almonds and pistachios, and stir until evenly combined. Spread the batter into the prepared tin and bake for about 1 hour 10 minutes, or until a skewer inserted into the centre comes out clean. Leave in the tin to cool for 15 minutes, then turn out onto a wire rack to cool completely.
For the ganache, place the chocolate in a medium bowl. Pour the double cream into a medium pan over a low heat. Add the salt, then bring to the boil and immediately pour the mixture over the chocolate. Leave for 5 minutes, then stir in the blackcurrant powder, if using, until the chocolate is smooth. Cool for 15 minutes, then refrigerate until set (about 1 hour), then whisk with an electric hand whisk, until light and smooth. Set aside.
Cut the cake into 3 equal rectangles. Place one rectangle on the cake board and spread it with a thick layer of ganache, then stack a second rectangle on top. Spread that one with a thick layer of ganache, too.
Cut the third rectangle in half through the thickness to give 2 thin sponge rectangles. Trim 1cm off of the long edge of one piece and place the largest part on top of the stacked sponges. Spread with ganache.
Cut the other thin rectangle into two lengths that are 4cm and 2cm wide respectively and sandwich these with ganache on top of the cake, spreading with another layer of ganache and topping with the remaining 1cm piece. You’re making a pyramid of cake pieces that decrease in size to a pointed roof.
Cover the whole cake thickly with the remaining ganache, making it as neat and smooth as you can. Chill for 30 minutes.
To decorate, dust the work surface lightly with icing sugar and roll out the sugar paste to about 3mm thick. To cover the sides, roll out 2 rectangles each measuring 12 x 20cm and place them against the sides of the cake. Trim with a knife to neaten. Roll another 4 rectangles measuring 12 x 20cm and place one against the front and one against the back of the cake. Use the remaining two as the roof panels. Use a cake icing smoother to smooth the surfaces.
Knead 30g of the sugar paste with a little yellow food colouring and use this to make the door. Dampen the back with a little water and fix it to the cake. Use a little of the white sugar paste to make steps up to the doorway.
To make the windows, colour about 150g of the sugar paste blue. Shape about 75g of the remaining white sugar paste into a barrel shape. Slice the barrel in half to make two bay windows. Dampen the back of each half and place them against the cake, either side of the door.
Shape the blue sugar paste into 2 small, rectangular windows and 2 rectangles to wrap around the bay windows. Dampen the backs and attach above and around the bays as appropriate. Add small, white sugar-paste windowsills.
To make the royal icing, put the egg white in the bowl of a stand mixer fitted with the whisk. Whisk for 10–20 seconds at medium speed, then turn off mixer, add some of the icing sugar and whisk at a slow speed. Gradually add the remaining icing sugar, increasing the mixer as the icing sugar is incorporated.
Use a spatula to wipe mixture from the sides of the bowl and add the lemon juice. Keep beating at high speed until you have a piping consistency.
Place the icing in the small piping bag and pipe the roof tiles, and the detail on the windows and door. Decorate the roof with silver balls or pearl dragées.