Preheat oven to 350 degrees.
Line cupcake tin with cupcake liners.
Blend cake mix, milk, melted butter, egg whites, and vanilla in a large bowl at low speed until moistened for about 30 seconds.
Beat at medium speed for about 2 minutes.
Spoon batter into cupcake cups about 2/3 of the way.
Place a mini peanut butter cup in each cupcake and cover with additional batter.
Bake for 17-20 minutes, until done. Check on the lower end of time to see if they are done.
Remove from cupcake tin and let cool completely on a wire rack before frosting.