Heat the oven to 180C/160C fan/gas 4 and line a 20cm square tin with baking parchment. Mix the flour, cocoa and sugar, breaking up any clumps of sugar.

Combine the yogurt, oil, syrup, vanilla and egg. Pour the wet ingredients into the dry and mix. Stir the baking powder into the coffee, pour this in and quickly mix to make a smooth batter (see tip, below). Pour into the tin and bake for 35 mins until a skewer inserted in the middle comes out with just a few crumbs attached.

Leave to cool in the tin for a few minutes, then transfer to a wire rack to cool completely. To make the frosting, melt the chocolate in 10-second bursts in the microwave. Beat in the syrup, vanilla, yogurt and 1 tbsp boiling water to loosen, if needed. Spread over the cake and leave to set for at least 30 mins, then cut into 16 squares. Dot over the walnuts, if using. Will keep for three days in an airtight container at room temperature, or up to five days in the fridge.