Once I figured out how much I needed to reduce the amount of butter in the cake recipe and how much peanut butter to add, I was well on my way to a delicious peanut butter cupcake.
1 1/4 cups all-purpose flour
1 1/2 teaspoons baking powder
Rounded 1/4 teaspoon salt
1/2 cup smooth peanut butter
1/2 stick (1/4 cup) unsalted butter, softened
1/2 cup packed light brown sugar
1 large egg
1/2 teaspoon vanilla
2/3 cup whole milk
Whisk together flour, baking powder, and salt in a bowl.
Beat together butter, and brown sugar in a large bowl with an electric mixer at medium speed until blended, about 2 minutes.