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Set aside 50g each of the peanut butter and chocolate. Heat oven to 180C/160C fan/gas 4 and line a 20cm square baking tin with baking parchment. Gently melt remaining peanut butter, chocolate and all the sugar in a pan, stirring occasionally, until the sugar has just about melted. Transfer mix into a bowl to cool down slightly. Turn off heat and use a wooden spoon to beat in the eggs one by one. Stir in the flour and scrape into the tin.
Melt reserved peanut butter in the microwave on High for 45 secs, or in a pan, until runny, then drizzle over the brownie. Bake for 20-25 mins until it has a crust, but the middle still seems slightly uncooked.
Melt reserved chocolate, drizzle over the brownie, then cool in the tin before cutting into squares.