Preheat oven to 350°F. Line muffin pan with baking cups. In a saucepan or microwave safe bowl, melt chocolate and butter; stir in vanilla. In medium bowl combine sugar, unbeaten eggs, flour, baking powder and salt. Stir in chocolate mixture. Fill cups half full; place peanut butter cup in center of each and top with remaining batter.
Bake 18-20 minutes or until toothpick inserted comes out clean. Cool in pan on cooling rack 8 minutes. Remove cupcakes from pan. Cool completely before icing.