In the bowl of a stand mixer fitted with the paddle attachment or a large mixing bowl using an electric hand mixer, cream together the butter, sugar and brown sugar until light and fluffy. Add the egg and vanilla, mixing until combined.
In a separate large bowl, whisk or sift together the dry ingredients. Gradually add the dry ingredients to the wet ingredients until just combined. Do not overmix.
Fold in the in M&Ms by hand. Cover and chill for 1-2 hours in the fridge.
Scoop 2 tablespoonfuls of batter, rolling it into about 2 inch balls. The batter will resemble playdough and should not be sticky after chilling. Chill the dough between batches.
Arrange the cookie balls on a baking sheet lined with parchment paper at least 2 inches apart. Don’t press down!
Bake for 10-12 minutes (the time varies due to different ovens).
Remove and cool on the sheet for 2-3 minutes before transferring to a wire cooling rack.
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