Preheat the oven to 350°F. Line 18 muffin cups with paper or foil bake cups.
Place the butter in large microwave-safe bowl. Microwave on HIGH 1 to 1 1/2 minutes or until melted. Stir in the sugar and vanilla. Whisk in the eggs and mix well. Add the cocoa; whisk until well-blended. Add the flour, baking powder and salt; mix well. Stir in 1 1/3 cups peanut butter chips. Divide the batter evenly into muffin cups (I like to use a spring scoop for ease in getting it in there!); sprinkle with the remaining 1/3 cup peanut butter chips.
Bake 20 to 25 minutes (check them at 20) or until the surface of the cupcakes is firm; cool completely in the pan on wire rack, then pop them out of the cupcake tin and store in a covered container.