- 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.