01 - Preheat oven to 350°F. Line a mini muffin tin with paper liners or grease each cup thoroughly with nonstick spray.
02 - In a medium mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until evenly distributed.
03 - In a separate bowl, whisk the melted and cooled butter, whole milk, egg, and vanilla extract until smooth and fully incorporated.
04 - Pour the wet mixture into the dry ingredients and stir gently with a spatula until just combined. Stop mixing as soon as no dry streaks remain to keep the muffins tender.
05 - Gently fold mini chocolate chips or fresh blueberries into the batter with a few light strokes, taking care not to overwork it.
06 - Spoon the batter evenly into the prepared mini muffin cups, filling each one about three-quarters full.
07 - Bake for 10 to 12 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are lightly golden.
08 - Let the muffins rest in the tin for 5 minutes, then transfer them to a wire rack to cool completely before serving or storing.