Preheat your oven to 350°F (175°C).
Line a mini muffin tin with paper liners or lightly grease with cooking spray.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt.
In another bowl, whisk together the melted butter, milk, vanilla extract, and egg until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix.
Gently fold in the rainbow sprinkles, distributing them evenly throughout the batter.
Divide the batter evenly among the mini muffin cups, filling each about 2/3 full.
Bake in the preheated oven for 10-12 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for a few minutes, then transfer them to a cooling rack to cool completely.