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Heat oven to 180C/160C fan/gas 4. Grease and line the base and sides of a 22cm square tin. Melt 85g butter and 50g chocolate together in a small pan. Mix 120g flour, 140g sugar, 3 tbsp of the cocoa and ½ tsp bicarbonate in a bowl.
Whisk together 1 egg with half the buttermilk (about 140ml). Scrape the melted chocolate mixture and egg mixture into the dry ingredients, along with 100ml boiling water. Whizz briefly with an electric whisk until lump-free. Scrape into the tin and bake for 18-20 mins until a skewer comes out clean.
Cool the cake in the tin for 15 mins, then transfer to a wire rack to finish cooling while you repeat steps 1 and 2 to make a second cake. Cool both completely.
To make the icing, melt the After Eights and cream together in a saucepan. Cool, then chill until spreadable.
Trim the cake edges, then halve to make 4 rectangles. Use half of the icing to sandwich the layers together, then spread the rest over the top and sides. Chill.
Knead some colouring into the ready-to-roll icing with the peppermint extract. Roll out on an icing sugar dusted surface, then lift over to cover the cake, smoothing with your hands. Trim excess icing, then scatter with decorations.
Chill again for 1 hr to firm up, then serve or keep in the fridge, removing 30 mins before serving.