Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square baking tin.
Combine the melted butter, granulated sugar and light brown sugar in a large mixing bowl or the bowl of a stand mixer. Combine on high speed for 3-5 minutes.
Add the eggs and vanilla and combine again.
Fold in the flour, baking powder, peanuts (if using) and 1 heaped tbsp peanut butter using a wooden spoon/silicone spatula.
Pour the batter into the prepared tin, and add on top about 3 heaped tbsp peanut butter. Swirl this into the batter gently using a knife/the end of a spoon - you want to see clear peanut butter.
Bake for 25-30 minutes, until golden on top and no longer wobbling in the middle. Let cool on a wire rack before slicing.
