1. Heat oven to 350°F. Line 18 muffin cups (2-1/2 inches in diameter) with paper or foil bake cups.
2. Place butter in large microwave-safe bowl. Microwave at HIGH (100%) 1 to 1-/2 minutes or until melted. Stir in sugar and vanilla. Add eggs; beat well. Add cocoa; beat until well blended. Add flour, baking powder and salt; beat well. Stir in 1-1/3 cups peanut butter chips. Divide batter evenly into muffin cups.
3. Bake 25 to 30 minutes or until surface is firm; remove from oven. Immediately sprinkle remaining 1/3 cup peanut butter chips over muffin tops, pressing in slightly. Cool completely in pan on wire rack. 1-1/2 dozen brownie cups.