Make the Cupcakes

1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. Combine the flour, baking powder, baking soda and salt in a medium sized bowl and set aside.
3. Add the butter, sugars and peanut butter to a large mixer bowl and beat together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
4. Add the sour cream and vanilla extract and beat until well combined.
5. Add the egg and egg yolk one at a time, mixing until mostly combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
6. Add half of the dry ingredients to the batter and mix until mostly combined.
7. Slowly add the milk and mix until well combined. The batter may look a little curdled, but that’s ok.
8. Add the remaining dry ingredients and mix until well combined and smooth. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Do not over mix the batter.
9. Fill the cupcake liners only about 1/2 full. The cupcakes rise a good bit. Bake for 13-17 minutes or until a toothpick inserted into the center comes out clean.
10. Remove cupcakes from the oven and place on a cooling rack to cool.

Make the Buttercream

1. Beat the butter in a large mixer bowl until smooth.
2. Add the peanut butter and mix until well combined.
3. Add about half of the powdered sugar and mix until well combined.
4. Add the vanilla extract, salt and 3 tablespoons of milk or water and mix until well combined.
5. Add the remaining powdered sugar and mix until well combined.
6. Add additional water or milk, as needed, to get the right consistency of frosting.
7. Pipe the frosting onto the cooled cupcakes. I used Ateco tip 847.
8. Sprinkle some crushed peanut butter chips onto the frosting, if desired.