Step 1

Preheat the oven to 350 degrees. Line 12 to 14 muffin cups with paper liners and set pans aside.

Step 2

Combine the water, butter, vegetable oil, and peanut butter in a small saucepan set over medium heat.

Step 3

Cook, stirring often, until peanut butter and butter are melted and mixture is smooth. Remove from the heat and let cool while you continue with the cupcake batter.

Step 4

Combine the flour, sugar, baking soda, and salt in a large mixing bowl and beat briefly with an electric mixer to combine.

Step 5

Add the sour cream, milk, egg, and vanilla and beat on low speed until combined.

Step 6

Beat in the cooled peanut butter mixture on low speed until batter is smooth, stopping mixer to scrape the sides and bottom of the bowl several times. Batter will be fairly thin.

Step 7

Divide batter evenly between the prepared muffin cups, filling each a little more than half full. Bake for 15 to 20 minutes, until a cake tester inserted into the center of the pans comes out clean.

Step 8

Cool in the muffin tins for 20 minutes, then transfer to a wire rack to cool completely before frosting.

Step 9

Once cupcakes are cool, make the frosting.

Step 10

Beat butter and peanut butter together until very smooth, then add 3 cups powdered sugar, the cocoa powder, 4 tablespoons milk, and salt and beat on low to combine.

Step 11

Scrape down the sides and bottom of the bowl, then turn mixer to medium and beat for a minute or two, until frosting is smooth.

Step 12

Add additional powdered sugar if frosting is too thin and additional milk if frosting is too thick. Frost cupcakes and decorate as desired.

Step 13

Uneaten cupcakes can be stored at room temp, covered, for up to 2 days or in the fridge for up to 3 days.

Step 14

They can also be frozen, tightly wrapped, for up to 2 months.

Step 15

Unfrosted cupcakes can be stored, tightly wrapped, at room temp for up to a day or so.