Preheat the oven to 180°C, fan 160°C, gas 4; line a 20cm x 30cm traybake tin with baking paper.

Put the butter, cocoa, caster sugar and Nutella into a large saucepan. Over a really low heat, gently melt together, stirring so that the cocoa and sugar don’t catch. Keep going until the butter is melted and the mixture has totally combined. Don’t worry about the sugar melting – it will look thick and gritty.

Turn off the heat, leave to cool for 5 minutes, then gradually beat in the egg with a wooden spoon. The mix will become shiny and smooth. Finally, stir in the flour and a pinch of salt. Set aside a quarter of the chopped nuts for the top, and stir the rest into the brownie batter.

Tip the batter into the prepared tin and smooth over the top. Sprinkle over the wafer pieces, then scatter over the reserved nuts. Bake on a middle-low oven shelf for 35-40 minutes – the brownie should be risen with a crisp top, but not completely cooked through. It will sink on cooling, and be quite gooey. Once cool, or just warm, slice into chunks. Great cold with a cuppa, but best served warm with ice cream.