Preheat your oven to 350°F and prepare a mini muffin pan by spraying it with non-stick cooking spray or lining it with paper liners.

In a large mixing bowl, combine the flour, sugar, baking powder, baking soda, and salt. Mix well to ensure the dry ingredients are evenly distributed.

In another bowl, whisk together the milk, egg, melted butter or oil, and vanilla extract until well combined.

Gradually add the wet ingredients to the dry ingredients, mixing gently until just combined. Be careful not to overmix; it's okay if there are a few lumps.

If you're adding mix-ins like chocolate chips or fruit, fold them into the batter at this stage.

Using a small cookie scoop or spoon, fill each mini muffin cup about ¾ full with the batter.

Bake in the preheated oven for 10-12 minutes, or until the muffins are lightly golden and a toothpick inserted in the center comes out clean.

Allow the mini muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.