Preheat the oven to 180C/350F/Gas 4. Line the bottom and sides of an 8x8 inch square baking pan with aluminium foil or parchment paper, and spray with baking spray. Set aside.

Whisk together the butter, sugar, salt, and cocoa powder. Stir in the vanilla extract, then mix in the eggs, one at a time, stirring vigorously after each one. Fold in the flour until fully combined, stirring until thick and shiny.

Pour the batter into the prepared pan, and spread out evenly. Bake for 30 - 35 minutes, or until a toothpick inserted into the centre comes out almost clean with a few moist crumbs. Allow to cool completely in the pan on a wire rack.

Place the butter and peanut butter in a saucepan over medium heat, and stir often until melted and combined. Remove from the heat and stir in the salt and vanilla extract. Gradually add the icing sugar, stirring until combined.

Spread the mixture on top of the cooled brownies, and smooth out evenly. Sprinkle some chocolate chips on top.

Cover and refrigerate for at least an hour, until the fudge has set and is no longer warm. Once set, remove from the pan, cut into squares, and enjoy!