Heat the oven to 190C/170C fan/gas 5 and put the kettle to boil. Oil four 20cm cake tins and line with baking parchment (alternatively, you can bake the sponges in batches if you only have two tins). Melt the butter in a small saucepan over a low heat, and continue to cook until foaming and the milk solids have turned golden brown, about 3-4 mins (watch closely so it doesn't burn). Once the butter smells nutty, remove from the heat and set aside until cooled slightly.

Combine the flour, baking powder, bicarbonate of soda and 1 tsp fine salt in a large bowl. Sift the cocoa powder into a separate bowl, then stir in the malt powder and sugar. Combine the eggs, soured cream, oil and vanilla extract in a jug. Slowly pour the wet ingredients into the cocoa mixture, whisking until smooth. Add the browned butter and mix again. Tip the chocolate mixture into the flour mixture, and mix until smooth and lump-free. Slowly pour 340ml boiled water from the kettle into the cake mixture, whisking until you have a smooth, glossy, slightly runny batter. Divide the batter equally between the tins and bake for 25-27 mins, or until a skewer inserted into the middle of each comes out clean. Leave to cool completely on a wire rack. (If reusing the tins, leave the cakes to cool in the tins for 15 mins, then turn out onto the wire rack before repeating the process.)

Meanwhile, make the ganache by melting the dark chocolate in a heatproof bowl set over a pan of simmering water, or in the microwave in short bursts. Add the soured cream to the bowl (keeping it over the heat), and mix until combined and glossy. Remove from the heat and leave to cool slightly.

In a separate mixing bowl, whip the double cream, vanilla extract, malt powder, maple syrup and salt to very soft peaks. Stir through the cooled sour cream and chocolate followed by half of the Maltesers®. Put one sponge on a serving plate or cake stand, spread over a quarter of the ganache, then top with a second sponge. Repeat with the remaining ganache and sponges, then top with the remaining Maltesers and a sprinkling of extra malt powder, if you like. Will keep chilled for up to three days.