Preheat the oven to 350°. Line two muffin tins with 18 paper cupcake liners. (If you only have one pan, you bake in batches.)
In a large mixing bowl, whisk together the flour, baking powder, baking soda, and salt.
In a medium mixing bowl, whisk together the light brown sugar, sour cream, peanut butter, milk, oil, egg, and vanilla extract.
Create a well in the flour mixture and pour in the peanut butter mixture. Whisk until completely combined. Divide the batter evenly between the paper liners (about ¼ cup per well).
Bake for 22 minutes or until a wooden toothpick inserted into the center comes out with a few moist crumbs. Let cool completely.
For the Peanut Butter Frosting:
In a large mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, combine the peanut butter and butter. Beat on medium-high speed until smooth and well combined, about 1 minute.
With the mixer on low speed, add the confectioners’ sugar, ½ cup at a time, until completely combined. Stop and scrape down the sides occasionally. Add the cream and vanilla. Increase speed to medium-high and beat until light and fluffy. Transfer the frosting to a piping bag and pipe onto the cooled cupcakes. Store cupcakes in an airtight container for up to 4 days.
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Notes
If you only have one muffin pan, you can bake the cupcakes in batches. Just let the first batch cool for a few minutes in the pan, then transfer them to a wire rack. Line again and bake as instructed.
Make sure you let the cupcakes cool completely before frosting, or the peanut butter frosting will just melt off the top of the cupcakes.
Make sure not to overfill the liners, as the cupcakes do rise quite a bit, and you don’t want them to overflow.
The egg should be at room temperature to ensure it incorporates into your batter evenly. If you forgot to take your egg out of the fridge ahead of time, you could quickly bring it to room temperature by placing it in a large bowl and covering it with warm tap water for 5 minutes.
Avoid overmixing as you risk over-developing the gluten in the batter leading to tough peanut butter flavored cupcakes.
Make sure to measure your flour correctly! Adding too much flour to the recipe is the most common mistake that will yield dry and dense cupcakes. The best and easiest way to measure flour is by using a scale. If you don’t have one, then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
You can top the frosting off with some fun garnish, such as a peanut butter cup, peanut butter chips, or chopped peanuts. A drizzle of chocolate ganache is a great garnish as well.
Make sure your brown sugar isn’t lumpy. See my post on how to soften brown sugar if your brown sugar has hardened or has lumps in it.