Preheat the oven to 400℉. Line muffin pans with paper liners and set aside.
In a large bowl, mix together the flour, sugar, baking powder, and salt until combined.
Make a well in the center of the flour mixture. Add in the butter and mix into the flour mixture using a fork or a pastry cutter.
Add in the eggs, milk, and vanilla extract, and mix until fully incorporated. Fold in the M&Ms.
Divide the mixture evenly into the muffin pans and bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Allow to cool in the pan for around 5 minutes before transferring to wire racks to cool completely.