Heat the oven to 180C/160C fan/gas 4 and line a 20cm square brownie tin with baking paper. Tip the butter and 250g of the chocolate into a small pan, place over a low heat and heat gently until melted and fully combined.

Add the sugar, eggs and vanilla bean paste to a large bowl and use electric beaters to whisk together until pale and really thick – it should leave a clear trail on itself when drizzled back into the bowl, 6-8 minutes.

Fold the melted chocolate and butter into the mix, then add the flour, cocoa powder and ½ tsp fine sea salt and fold until evenly incorporated. Scrape into the tin and smooth the top. Scatter with the remaining chopped dark chocolate and bake for 20-25 minutes until baked around the edges, but still with a little wobble in the middle. Cool and then cut into 12 pieces.