Combine peanut butter sugar, oil, and peanut butter and stir well to combine. Beat in eggs, then stir in milk and vanilla extract.
In a separate bowl, whisk together flour, baking powder, and salt.
Gently combine wet and dry ingredients with spatula.
Stir in chocolate chips when dry ingredients are halfway combined (don't over-mix the batter or muffins will be dense and dry).
Divide batter into muffin liners, filling each cavity ¾ the way full (keep in mind this recipe makes 18 muffins, so you will need two tins. If you only have one, bake the first batch and cover the remaining batter until the first batch has baked and mostly cooled). If desired, sprinkle muffins with additional granulated sugar.
Bake on 375F (190C) for 20-22 mins or until a toothpick inserted in the center comes out clean (avoid chocolate chips) or with moist crumbs. Do not over-bake or they will be dry.
¹You may substitute whole milk for the buttermilk in this recipe without making any other changes. I prefer the buttermilk because it makes the muffins slightly more moist and more flavorful.Store muffins in an airtight container at room temperature for up to 2 days.