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In a medium mixing bowl, combine: cocoa powder, flour, baking powder, baking soda and salt, whisk and set aside.
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In a large mixing bowl or in the bowl of an electric mixer fitted with the whisk attachment, combine peanut butter, oil, and brown sugar, beat until all combined and fluffy.
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Add in the eggs one at a time, and vanilla extract. Beat on medium speed until combined. Stop a few times to scrape the sides and bottom of the bowl with a spatula.
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Add the flour mixture and buttermilk, alternating between the two and mixing by hand (to avoid over mixing) until all combined.
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Fill the cupcake liners almost completely full with batter.
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Bake for 15-17 minutes or until a toothpick inserted in the center comes out with just a few crumbs.
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Allow the cupcakes to cool for 3 minutes in the pan, then transfer them to a cooling rack. Place cupcakes upside down on the cooling rack to create a dome top.
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Allow the cupcakes to cool completely before frosting them.