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The uniced sponges can be frozen for up to 1 month. Once iced and cut, the cake keeps for up to 4 days in an airtight container.
Preheat the oven to 180°C, fan 160°C, gas 4. Grease 2 x 20cm round cake tins and line the base and sides with baking paper.
Using a stand mixer (or electric beaters), beat the butter, sugar and salt together until pale and fluffy. Gradually add the eggs, one by one, until fully incorporated. If the mixture starts to look like scrambled eggs, add a little of the flour.
Beat in the buttermilk, then the flour and malted milk powder until combined. Tip in the chocolate chips; beat again until just mixed through. Divide the mixture equally between the tins (we use scales to do this – about 650g in each tin) and bake for 30-40 minutes until well risen and golden. A skewer should come out clean when inserted into the middle of the cakes. Remove from the oven and leave to cool in the tin for 10 minutes before turning out on a wire rack to cool completely.
To make the buttercream, beat the butter and half the icing sugar together until smooth and fluffy. Add the rest of the icing sugar, the malted milk powder, vanilla and 1 tablespoon of milk. Beat until smooth. Add the remaining milk for a fluffy consistency, if needed.
Put a little buttercream on the base of a cake board or stand. Stick one sponge to the base. Spoon over a generous heap of buttercream, working it across the middle until evenly coated. Sandwich the other sponge on top of the cake; coat the entire cake in a thin layer of buttercream. Work out from the middle, pushing the icing down the sides using a palette knife. Chill for 10-20 minutes.
Once the first layer of icing has set, apply a second coat, using a large palette knife. When coated, you can add a swirl to the top of the cake, working back and forth with the palette knife to create this effect. Decorate with extra cookies, if using; serve at room temperature.