Heat the oven to 180C/160C fan/gas 4. Butter a deep 20 x 30cm rectangular baking tin and line with baking parchment. Tip 150g of the hazelnuts into a food processor and blitz until finely chopped, then tip in the flour, baking powder and a pinch of sea salt, and blitz again until blended as finely as possible.

Tip the softened butter and brown sugar into a bowl and beat using an electric whisk until combined. Beat in the caramel, coffee and vanilla, then add the eggs, one at a time, beating between each addition. Beat in the dry ingredients until everything is incorporated, then scrape the batter into the prepared tin and level the surface using a spatula. Bake for 30-35 mins, or until the top springs back when gently pressed and a skewer inserted into the middle comes out clean. Cool completely in the tin.

To make the icing, beat the caramel, butter and icing sugar together in a bowl using an electric whisk until smooth, then add the coffee. If the icing is very loose, chill for 30 mins until firm but spreadable. Turn the cooled cake out onto a large plate or board, then spread over the icing using a small offset palette knife. Roughly chop the remaining 50g hazelnuts and scatter these over the top, then cut the cake into 16 rectangles or 32 squares. Will keep in an airtight container for up to three days.