Cream butter, brown sugar, and granulated sugar together in a large bowl with a stand mixer or hand mixer on medium speed.
Mix in the peanut butter until incorporated.
Add the egg and mix for a few seconds.
Add the vanilla and mix for a few seconds until incorporated.
Slowly mix in the baking soda, flour, and 1/4 tsp salt (if using unsalted butter) just until incorporated.
Fold in the M&M's with a wooden spoon
Chill the dough in the refrigerator for at least 30 minutes.
Preheat oven to 350°. Line a baking sheet with parchment paper or a silicone baking mat.
Using a tablespoon sized cookie scoop, drop dough onto cookie sheet. Press down on dough balls with a drinking glass to flatten slightly.
Bake for 7-10 minutes, until the bottoms are slightly golden brown.
After cooling on the cookie sheet for 2 minutes, use a spatula to transfer cookies to a cooling rack to cool completely.
Notes
This dough may be frozen to be baked at a later time. After scooping out the dough and flattening it, put the cookie sheet in the freezer for 15 minutes to firm up the dough. Then transfer to a freezer storage bag. When ready to bake, there is no need to thaw them. Just add 1-2 minutes to the baking time. If you are using salted butter, do not add extra salt in step #5.
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