Preheat the oven to 350°F. Prepare two baking sheets with parchment paper or a silicone baking mat.
Stir together the peanut butter and sugars until combined well.
Add the egg, vanilla, baking soda, and salt. Stir until combined.
Roll the cookie dough into 16 balls. The balls should be about 1 1/2 inches (in diameter) or 1 ounce (28-30 grams) each. Divide them between the baking sheets, spreading them out to keep the cookies separated for baking.
Use the tines of a fork to flatten the cookies. Press 3 peanut butter M&M's into each cookie.
Bake for 10 minutes to lightly brown the cookies. Let them cool on the pan.
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