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

Set aside 2 tablespoons of peanut butter for the swirls. Warm the remaining peanut butter in the microwave to loosen (~30 sec on high), then pour into the pan. Spread evenly, sprinkle with half 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 sugar, mix, then add eggs and vanilla. Mix well until smooth. Add flour, cocoa, and salt, stir until smooth.

Pour half the batter into the pan. Top with frozen peanut butter slab, then cover with remaining batter.

Dollop/drizzle the reserved 2 tablespoons of peanut butter across the surface. Use a knife or teaspoon to create swirls.

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

Cool in the pan on the counter for 1 hour, then refrigerate for at least 3 hours to set. Cut into 16 squares.