Preheat your oven to 375°F (190°C) and lightly grease a mini muffin tin with non-stick cooking spray.

In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In a separate bowl, whisk together the milk, yogurt, egg, melted butter, and vanilla extract until fully combined.

Gradually add the wet ingredients to the dry ingredients and mix until just combined. Avoid overmixing; the batter should be slightly lumpy.

If desired, gently fold in the mini chocolate chips or other mix-ins.

Spoon the batter evenly into the prepared mini muffin tin, filling each cup about 3/4 full.

Bake for 10-12 minutes, or until the tops are lightly golden and a toothpick inserted into the center of a muffin comes out clean.

Allow the muffins to cool in the tin for 2-3 minutes, then transfer to a wire rack to cool completely. Serve warm or at room temperature.