In a large bowl, beat cake mix, milk, 1/2 cup peanut butter, vanilla, and eggs until well blended.
Gently stir in chopped peanut butter cups.
Pour batter evenly into two greased loaf pans.
Bake at 350 for 40 - 45 minutes or until a cake tester comes out clean.
Cool completely on wire racks.
For the frosting, melt the remaining 1/4 cup peanut butter and chocolate chips in a small saucepan over medium low heat stirring constantly until smooth.