20cm round deep cake tin, greased and base-lined with non-stick baking paper; baking sheet lined with non-stick baking paper
Heat the oven to 160°C/140°C fan/gas 3. To make the sponge, gently heat the 125g butter, golden syrup, and both sugars in a large heavy-based pan until melted and smooth. Remove from the heat, add the 50g chocolate, stir until melted, then set aside to cool. When the mixture has cooled slightly, beat in the eggs using a wooden spoon
Stir together the remaining sponge ingredients including the evaporated milk in a large mixing bowl, then add the chocolate mixture and fold in using a flexible spatula. Pour the batter into the prepared tin, then bake for 50-55 minutes until well risen and a skewer pushed into the center comes out clean. Cool in the tin for 15 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, make the decoration: remove the crystallized stem ginger from the fridge and put it in a large mixing bowl with the melted chocolate. Using 2 forks, toss the ginger in the chocolate to coat, then transfer each piece to the lined baking sheet and leave to set.
For the icing, beat the 75g butter in a mixing bowl until very soft using an electric mixer. Working in batches, mix in the icing sugar, then the evaporated milk and ground ginger, then beat again until light and fluffy. Add the melted chocolate to the bowl and mix again.
Top the cake with the icing in great swathes and swirls, then scatter over the chocolate-coated stem ginger. To serve, sprinkle generously with icing sugar to create a snowy scene.
The decorated cake will keep in an airtight container for up to 1 week. The un-iced sponge will keep, well wrapped, in the freezer for up to 1 month. Defrost overnight, then decorate to serve.