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