Preheat the oven to gas 4, 180°C, fan 160°C.

Grease and line a 26 x 17cm baking tin with nonstick baking paper.

Start by making the cookie layer. Beat the butter and sugar in a mixing bowl with an electric whisk for 3-4 mins until light and fluffy.

Whisk in the egg and vanilla until just combined.

In a separate bowl, mix together the flour and bicarbonate of soda, then fold into the butter mixture, followed by the peanuts, until it just forms a dough.

Transfer to the prepared tin; lightly press in with the back of a spoon to cover the bottom with no gaps. Chill for 30 mins.

Meanwhile, make the brownie layer. Melt the butter and chocolate in a heatproof bowl set over a pan of barely simmering water (don’t let the water touch the bottom of the bowl). Stir briefly; remove from the heat and leave to cool for 10 mins.

Whisk the eggs and sugar in a mixing bowl with an electric whisk for 3-4 mins until pale and fluffy.

Fold in the melted chocolate, then sieve in the flour, cocoa and baking powder; fold in until just combined.

Pour the mixture on top of the cookie base and dollop the peanut butter over the top (if it’s very stiff, warm it gently in a small pan first).

Use a cutlery knife to swirl it into the brownie mixture.

Bake for 40-45 mins until a skewer inserted in the centre comes out with just a few crumbs, and it still feels soft on top.

Leave to cool completely in the tin before cutting into 16.