01 - Preheat oven to 350°F (175°C). Line a mini muffin tin with paper liners or grease thoroughly with nonstick spray.
02 - In a medium bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until evenly distributed.
03 - In a separate bowl, whisk together the melted butter, milk, egg, and vanilla extract until smooth and fully incorporated.
04 - Pour the wet ingredient mixture into the dry ingredients and stir gently until just combined. Be careful not to overmix, as this can produce tough muffins.
05 - Gently fold in the mini chocolate chips or blueberries, if using, distributing them evenly throughout the batter.
06 - Spoon the batter evenly into the prepared mini muffin tin, filling each cup 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.
08 - Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.