Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, whisk melted butter with brown sugar, peanut butter, eggs, and vanilla extract until smooth.
Add in the flour, salt, and baking powder and mix until smooth. Evenly divide the batter between the paper lined muffin pan. Bake for 12-15 minutes.