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Heat oven to 190C/170C fan/gas 5 and line a 20 x 30cm cake tin with baking parchment. Melt the butter in a pan with the sugar, syrup and coffee. Stir in the walnuts, porridge oats and flour with a pinch of salt. Press evenly into the tin and bake for 25 mins, then leave to cool.
Meanwhile, make the icing. Melt the butter into the coffee, then stir into the icing sugar to make a smooth icing. Once the flapjacks have cooled to room temperature, spread over the icing. Carefully space out your walnut halves, then scatter over the finely chopped walnuts to finish. Leave until the icing is set and firm – you can pop in the fridge to speed up this bit if you like. Will keep in an airtight container for a week.