Preheat your oven to 350°F (175°C) and grease a mini muffin tin.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
In another bowl, beat the egg, then add milk, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Gently fold in your choice of mix-ins.
Fill each muffin cup about two-thirds full with batter.
Bake for 12-15 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and let cool for a few minutes in the tin.
Transfer muffins to a wire rack to cool completely.
Enjoy your fresh mini muffins warm or at room temperature!