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Butter a 30 x 20cm 2.5cm deep cake tin and line with parchment. Heat the oven to 180C/160C fan/ gas 4. Dissolve the coffee in 3 tbsp of boiling water and set aside. Scatter all the pecans, including the 8 for the topping, over a baking tray and toast in the oven for 8-10 mins, then leave to cool slightly. Split 8 of the pecans in half to make 16 pieces and set aside. Chop the rest finely, retaining a bit of texture.
Use an electric whisk to cream the butter and sugar together in a bowl, then crack in and incorporate the eggs, one at a time. Beat in half the dissolved coffee. When everything is mixed together evenly, fold through the flour, baking powder, a pinch of salt and half the chopped nuts. Scrape into the tin and scatter over the rest of the chopped nuts, then level the top with a spatula. Bake for 30 mins or until a skewer inserted into the middle comes out clean, then leave to cool completely.
To make the icing, beat the butter until smooth, then beat in the icing sugar, the vanilla and the remaining dissolved coffee. Lift the cooled cake onto a board and spread with the icing. Top with the pecan halves, cut into squares and serve.