Add the softened butter to a mixing bowl or bowl of a stand mixer and beat with both sugars for about 1 full minute.
Add in the egg and vanilla and beat until well combined.
Spoon and level the flour to measure, then add to the bowl with the baking powder, baking soda and salt.
Beat until well combined.
Fold in the m&ms.
Place in the fridge to chill for 1 hour.
After about 45 minutes, preheat the oven to 350 degrees Fahrenheit.
Scoop the dough using a 2 tablespoon scoop and place on a baking sheet lined with parchment paper.
Bake for 13-18 minutes or until the edges are just golden brown. They will continue to harden as they cool.
As soon as you remove them from the oven, take a round cookie cutter and swivel it around the edges to shape it into a perfect circle. This is optional, but helps make them look nice!
Let cool completely, then transfer to a wire rack and enjoy!