Preheat oven to 350 degrees F. Line 2 regular muffin tins with cupcake liners.
Mix together the cake mix, milk, eggs, oil and Greek yogurt until smooth. Spoon batter into cupcake pans, filling the liners 1/2 full.
Place a Reese's PB cup into the center of the batter in each cup (it's okay if the the tops of the Reese's are showing, like photo above).
Bake cupcakes for 16-20 minutes or until done. Allow to cool completely before frosting.
For the Peanut Butter Frosting
Beat together butter, and peanut butter and mix until smooth.
Add powdered sugar and vanilla and beat again until combined.
Add milk and mix until light and fluffy. (Add more milk or powdered sugar if needed to make the frosting thicker or thinner).
Frost cupcakes (I used a wilton 1M tip in the photos above).
Chop up the remaining pieces of Reese's and gently press into frosting.
Store leftover cupcakes, covered, in the fridge for 3-4 days.
*For a completely "from scratch" chocolate cupcake recipe, here's my favoriteTo make ahead:Bake and cool cupcakes 1 day in advance. Keep them covered tightly at room temperature and frost the day of serving.Unfrosted cupcakes can also be frozen up to 2 months. Thaw overnight in the refrigerator and bring to room temperature before frosting and serving.