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Preheat the oven to 350 degrees. Line 12 to 14 muffin cups with paper liners and set pans aside.
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Combine the water, butter, vegetable oil, and peanut butter in a small saucepan set over medium heat.
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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.
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Combine the flour, sugar, baking soda, and salt in a large mixing bowl and beat briefly with an electric mixer to combine.
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Add the sour cream, milk, egg, and vanilla and beat on low speed until combined.
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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.
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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.
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Cool in the muffin tins for 20 minutes, then transfer to a wire rack to cool completely before frosting.