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Preheat oven to 350 degrees. Line two baking sheets with parchment paper.
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In a heatproof bowl over simmering water or in the microwave, melt the butter and chopped chocolate. Stir until smooth, then let it cool slightly.
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In a separate mixing bowl, whisk together the granulated sugar, brown sugar, eggs and vanilla extract until thick and pale. Slowly pour the cooled chocolate mixture into the egg mixture and stir until well combined.
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Sift in the flour, cocoa powder, baking powder and salt. Fold gently with a spatula until a thick, glossy batter forms.
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In another bowl, mix the peanut butter with powdered sugar until smooth. Scoop tablespoons of brownie batter onto the prepared baking sheets. Drop half a teaspoon of peanut butter mixture on each and swirl gently with a toothpick.
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Bake for 10-12 minutes, until tops are crackly and centers are soft. Cool on the baking sheet for 5 minutes, then transfer to a wire rack. Enjoy warm or store in an airtight container.
