Preheat the oven to 350 degrees F. Insert liners into a medium cupcake pan.
In a large bowl cream together the butter, peanut butter, and brown sugar with an electric mixer on medium speed until fluffy, about 3-5 minutes. Add the eggs. Beat well. Mix in the vanilla.
In a separate bowl, combine the flour, baking powder, and salt.
Add the dry ingredients to creamed mixture, alternating with the milk. Mix well.
Fill the cupcake liners 1/2 to ¾ full. Bake for 15-20 minutes or until a toothpick inserted in the center of cupcakes come out clean. Cool cupcakes in the pan.
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