Line 18 to 20 cups of a standard muffin tin with bake cups. Combine the filling ingredients in a medium-size bowl and beat them with an electric mixer until evenly blended. The texture should be like whipped peanut butter. If it's not stiff enough, add a little extra peanut butter
Fill each muffin cup about halfway with brownie batter. Spoon about 2 teaspoons of peanut butter filling into the center, pushing it down slightly into the batter. Cover the filling with another spoonful of batter, making sure that the cups are no more than about two thirds to three quarters full
Bake for 15 to 17 minutes, just until well risen and cracked on top. Then transfer the pans to a wire rack and cool for 30 minutes. Remove the brownies from the pan and place them directly on the wire rack to finish cooling (they'll sink as they do) before serving. Makes 18 to 20 Peanut Butter Cup Brownies.