Line 18 to 20 cups of muffin tin with bake cups. Combine filling in medium bowl and beat with hand mixer until evenly blended. The texture should be like whipped peanut butter. If not stiff enough add more peanut butter.
Fill each muffin cup about halfway with brownie mix. Spoon about 2 tsp. peanut butter filling into the center, pushing it down slightly into batter. Cover the filling with another spoonful of batter, making sure the cups are no more than about 2/3 to 3/4 full.
Bake at 350 for 15 to 17 minutes just until well risen and cracked on top. Cool for 30 minutes,(they'll sink).