Heat the oven to 180C/160C fan/ gas 4. Butter and line the base and sides of a rectangular 20 x 30cm tin with baking parchment.

To make the base, blitz the walnuts in a food processor until finely chopped. Add the flour, sugar, butter and 1/2 tsp salt and blitz until the mixture is combined. Tip onto a work surface and bring together into a dough. Press the dough into the base of the tin and bake for 25 mins until lightly golden. Leave to cool.

To make the caramel, mix the espresso powder in a small bowl with 1 tbsp boiling water to dissolve. Tip the sugar, condensed milk and espresso into a heavy-based saucepan. Bring to a simmer, stirring to dissolve the sugar. Once the sugar has dissolved, boil vigorously for 3 mins, whisking continuously until the colour has darkened slightly and the caramel has thickened. Leave to cool for 20 mins, then pour the caramel over the biscuit layer and chill in the fridge for 2 hrs.

To make the topping, heat the dark chocolate and butter in a heatproof bowl in the microwave in 30 second bursts until evenly melted. Pour over the caramel layer. Melt the white chocolate in the same way and mix with the espresso powder. Swirl and drizzle the white chocolate over the dark chocolate layer. Scatter over the coffee beans and walnuts. Chill for a further 2 hrs or overnight to set before slicing into squares.