Preheat the oven to 180℃ fan/200℃/Gas Mark 6.
Line a baking tray with baking paper.
Melt the butter in a saucepan on a very low heat.
When it's starting to melt, add in the dark chocolate and milk chocolate and the sugar and stir until it's all melted together.
Once they are melted remove from the heat and leave to cool for 5-10 minutes.
Beat in each egg, one by one.
Stir in the flour.
Pour into your baking tin and top with the peanut butters (see notes).
Using a knife, swirl the mixture so that some of the batter is on top of the peanut butter and some remains on top, making a pretty pattern.
Bake for 30 minutes, or until the edges are just solid but the middle still has a little wobble. Don't overbake them!
Ovens do vary so don't be alarmed if you recipe takes that little bit longer. Brownies are better under-done than over.
Allow to cool COMPLETELY before cutting up.
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