Allow butter and eggs to stand at room temperature for 30 minutes. Meanwhile, line thirty-four to thirty-six 2-1/2-inch muffin cups with paper bake cups. In a medium bowl combine flour, baking powder, and salt. Set aside.
Preheat oven to 375 degrees F. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add peanut butter; beat until combined. Gradually add brown sugar and granulated sugar, about 1/4 cup at a time, beating on medium speed until combined. Scrape sides of bowl; beat for 2 minutes more or until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Alternately add flour mixture and milk to butter mixture, beating on low speed after each addition just until combined.
Place half of the batter into a separate mixing bowl; add melted chocolate. Beat with electric mixer on low speed just until combined.
Fill prepared muffin cups by alternating spoonfuls of peanut butter batter and chocolate batter, filling each cup about two-thirds full. Use a butter knife or skewer to swirl batter.
Bake 15 to 18 minutes or until a toothpick inserted in centers comes out clean. Cool cupcakes in muffin cups on wire racks for 5 minutes. Remove cupcakes from muffin cups. Cool completely on wire racks.
In a pastry bag fitted with a star or round tip place Peanut Butter Frosting side-by-side with the Chocolate Frosting. Pipe frostings onto cupcakes. Top with shaved chocolate and/or peanut butter cup halves. Makes 34 to 36 (2-1/2 inch) cupcakes.
Peanut Butter Frosting
Allow cream cheese to stand at room temperature for 30 minutes. In a large mixing bowl beat cream cheese, peanut butter, and vanilla with an electric mixer on medium speed until light and fluffy. Gradually beat in powdered sugar. Beat in enough milk, 1 teaspoon at a time, until frosting reaches piping consistency. Divide frosting into two portions. Set one portion aside for Chocolate Frosting (see recipe, below).
Beat the Peanut Butter Frosting and milk chocolate with an electric mixer on medium speed until combined. If necessary, beat in additional powdered sugar or milk until frosting reaches a piping consistency. Makes about 1-1/2 cups.