• Preheat the oven to 350F (180C). Line an 8x8 inch (20x20 cm) pan with parchment paper so that there is an overhang around the edges. You may want to lightly grease the pan first so that the parchment sticks to the pan.

Peanut Butter Filling

• In a medium bowl, beat together the peanut butter, powdered sugar and salt until combined. If your powdered sugar is lumpy, sift first. Place in the fridge as you make the brownie batter.

Brownie Batter

• In a small bowl, whisk together the flour, cocoa and salt. Set aside.
• In a large bowl, melt together the butter and chocolate. If using bar/blocks of chocolate, chop finely first. Microwave on medium power (not high power, which is often the default) for 45-second intervals, stirring between each interval. Alternatively, melt in a double boiler over gently simmering water.
• Let the mixture sit for about 5 minutes to cool slightly. Then whisk in the sugar, 2 eggs, additional egg yolk and vanilla extract until combined and the mixture is glossy (you shouldn't see clumps of egg).
• Add the dry ingredients into the chocolate and butter mixture (sift them in if they're lumpy). Then whisk until the batter is smooth and you no longer see streaks of dry ingredients.

Assembling and Baking the Brownies

• Add ½ of the brownie batter to the prepared tin and smooth into an even layer.
• Take the peanut butter filling out of the fridge. Scoop out tablespoons and flatten them down slightly. Then place the peanut butter pieces on top of the brownie layer to create an even layer that covers the brownie (if it doesn't quite cover all of the brownie, that's ok.
• Spoon the remaining brownie batter over the peanut butter layer - do not just pour the remaining brownie batter on top as you can end up disturbing the peanut butter layer.
• Spread the top brownie batter layer into an even layer so that the peanut butter layer is covered.
• Baking the brownies in the middle of the preheated oven for about 32-37 minutes, or until the top looks set and an inserted toothpick comes out clean or with a few damp crumbs. It shouldn't have gooey any brownie batter on it.
• Cool the brownies in the pan until the pan is no longer warm to the touch. This will take a few hours. You can speed it up by placing the brownie pan in the fridge on a wire rack. When ready to slice, lift out of the pan using the overhang of the parchment paper. Set on a cutting board and slice using a sharp knife.