Grease a 20cm/8" square pan with butter and line with baking paper.

Set aside 2 tablespoons of the peanut butter for swirls. Warm the remaining peanut butter in the microwave to loosen (~30 sec on high), then pour into the pan, spread evenly, and sprinkle with half of the salt flakes. Freeze for 2 – 3 hours until firm.

Preheat oven to 180°C/350°F (160°C fan forced).

Place butter and chocolate in a heatproof bowl. Microwave in 30-second bursts, stirring in between until melted and smooth.

Add brown sugar to the melted mixture and mix. Then add eggs and vanilla, mixing until smooth. Add flour, cocoa, and salt, then stir until smooth.

Pour half the batter into the pan. Top with the frozen peanut butter slab and spread with remaining batter.

Randomly dollop/drizzle the reserved 2 tbsp peanut butter across the surface and swirl using a knife or teaspoon.

Bake for 32 minutes. The middle will still jiggle but the edges should be firmish.

Cool in the pan on the counter for 1 hour, then refrigerate for at least 3 hours to set. Cut into 16 squares and let come to room temperature for 15 minutes before serving.