Nutrition: per slice (between 10)
First, make the ganache topping. Put the chopped chocolate in a heatproof bowl. Warm the double cream and Horlicks powder in a small pan over a low heat, whisking until just steaming. Pour this over the chocolate. Leave to stand for 3 mins so the chocolate melts, then whisk together the mixture until you have a shiny, glossy ganache. Cover and chill for 1-2 hrs.
Heat the oven to 180C/160C fan/gas 4. Butter a 33cm x 22.5cm x 5cm traybake tin and line with baking paper. Put the butter and golden caster sugar in a large bowl, and beat together using an electric whisk until pale, light and fluffy. Add the vanilla, eggs and soured cream, and beat again until smooth and combined.
Sift in the cocoa powder, flour, baking powder, bicarb and Horlicks powder. Fold together until the mixture is smooth and no pockets of flour remain. Scrape into the tin and smooth the surface. Bake for 25 mins until a skewer inserted into the middle comes out clean. Leave to cool in the tin for 10 mins, then gently invert onto a wire rack and leave to cool completely.
Once the cake and ganache have both cooled, whip the ganache using an electric whisk until it just holds a soft peak – be careful not to over-whip it. Spread the whipped ganache over the cake using a palette knife, then scatter over the chopped Maltesers and chocolate eggs before cutting into squares to serve.
