Preheat the oven to 350°F. Line an 8 inch square pan with parchment paper and set aside.
In a medium bowl, cream together the butter and sugar until light and fluffy. Mix in the egg and vanilla until fully combined.
Mix in the flour until fully incorporated. Fold in the M&Ms.
Press into the prepared baking pan and bake for 20-25 minutes, or until lightly brown. Remove from oven and allow to cool completely in the pan before cutting into squares