Method

1. Butter the base and sides of both 20cm square cake tins and line with baking parchment. Melt the dark chocolate in a heatproof bowl in the microwave in 30-second bursts, or in a bowl set over a pan of simmering water, ensuring the bowl doesn’t touch the water. Stir until smooth. Leave to cool completely.

2. Meanwhile, heat the oven to 180C/160C fan/gas 4. Beat the butter, sugars and vanilla together using an electric whisk or a stand mixer until pale and fluffy, about 4-5 mins. Add the eggs, one at a time, beating well after each addition and scraping down the side of the bowl. Don’t worry if it looks curdled – it will come together when the dry ingredients are added.

3. With the whisk or mixer on low, pour in the melted, cooled chocolate, to incorporate, then add the yogurt, mixing on low for about 20 seconds. In a separate bowl, stir the spices with ½ tsp salt, the baking powder and flour, then sieve in the cocoa and mix in. Tip the dry ingredients into the wet in three additions, mixing on low until just combined. Don’t overbeat or the sponge will be dense.

4. Divide the batter between the tins, level the tops using the back of a spoon and bake for 28-35 mins, or until a skewer inserted into the middles comes out slightly pasty (not clean) and the sponges are shrinking away from the sides of the tins a bit (check after 25 mins to be safe). Leave to cool in the tins for 10 mins, then turn out onto a wire rack, remove the parchment and leave to cool completely. Will keep in an airtight container at room temperature for up to two days.

5. Prepare the vanilla spiced biscuit cookies decorations. For the treacle buttercream, beat the butter using an electric whisk or stand mixer until pale and creamy, then add the icing sugar in three additions – mix in slowly at first, then turn up the speed and beat until pale and fluffy each time. Beat in the spices and treacle until combined. Cover the bowl with a damp tea towel and set aside (this will stop it from developing a skin). Will keep covered and chilled for up to three days. If making ahead, remove from the fridge at least 1-2 hrs before using so it can come back up to room temperature.

6. Split the cooled sponges through the equator using a cake leveller or large serrated knife so you have four layers. Put one of the sponge layers on a cake board that’s large enough to hold the finished house, but will still fit in the fridge. (This is quite a delicate sponge, so it’s a good idea to use a cake lifter, palette knives or a small baking sheet when you’re moving it all.) Spoon over 200g buttercream and smooth out using a palette knife so it reaches the edges. Sandwich with another sponge layer, making sure it lines up neatly, then press down gently so a little of the buttercream pushes out of the sides.

7. Smooth another 200g buttercream over the top, then sandwich with another sponge layer, then another 200g buttercream and the final sponge layer. Make sure the cake is all in line, then smooth over the buttercream that’s been pushed out between the layers so it covers the sides and seals in the crumbs. Chill for about 45 mins until completely firmed up. (You can also put it in the freezer for slightly less time if you want to speed this up.)

12. Chill the cake for 30 mins, then smooth a thin layer of buttercream over the whole cake to fully seal the sponge (about 200g buttercream). Try not to get crumbs in the rest of the buttercream, as you’ll use this to decorate the sides later. Chill the cake for another 1 hr, then smooth another 200g buttercream over the sides and ends (don’t cover the roof yet), using a cake smoother or palette knife to ensure it’s as neat as possible. Chill for another 30 mins.

14. Mix the royal icing sugar with just enough water to make a thick icing. Spoon or pipe the icing over the roof and along the eaves so it looks snow-capped. You can also create small icicles hanging from the roof eaves, if you like, by piping a small blob on the edge of the roof, then carefully dragging the piping bag downwards as you pipe, before releasing. Decorate the house with the cookie windows, door, star and wreath (using the vanilla spiced cookies recipe), pushing them into the buttercream or icing. If you find the windows are sliding, secure with a couple of cocktail sticks to hold them in place while setting. Leave the whole cake to set for a couple of hours at room temperature, then remove any cocktail sticks you’ve used to secure the roof or decorations before serving. Will keep in a cool place for up to five days. Keep the cut surface well covered.