1Preheat the oven to 350F.
2In a large bowl, beat the butter and sugars until pale and light; beat in the eggs and milk. Don't worry if the mixture looks like it's separating. Add the flour, baking powder and salt and stir (or beat on low) until combined. Remove about a cup an put it into another bowl.
3Stir the cocoa and chocolate chips into the original bowl, and the peanut butter into the batter you put in another bowl. Drop the chocolate batter in big dollops in a parchment-lined (or greased) 8x8-inch pan, and drop the peanut butter batter in spoonfuls in the spaces between. Drag the tip of a knife or a bamboo skewer through both, creating a swirled effect.
4Bake for 20-25 minutes, until the edges start to pull away from the sides of the pan, but the brownie is still soft in the middle. Cool in the pan on a wire rack.