Preheat the oven to 350 F. In a large bowl, mix together butter and powdered sugar until fluffy. Add in vanilla extract and mix until smooth. Add in the flour and mix until it is fully combined.
Fold in the M&Ms. Roll the cookie dough into balls and flatten slightly. Place the cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.