Preheat oven to 350 degrees F. Line 2 baking sheets with parchment paper or silpat mats and set aside.
In a medium bowl whisk together the flour, baking soda and salt until combined.
With a stand mixer or electric hand mixer in a large bowl, beat together the butter and sugars until smooth and creamy, about 3 minutes. Add egg and vanilla extract and beat on high until combined- be sure to scrape the sides of the bowl. Slowly add the dry ingredients to the wet ingredients and mix together on low, scraping down the sides of the bowl. Stir in the mini M&Ms and mix again to combine.
Form the cookie dough into small tablespoon balls and place on prepared baking sheet, about 1 1/2 inches apart.
Optional: press more M&Ms on the top of the cookie to make them look extra colorful.
Bake for 8-10 minutes or until the edges are golden brown. Allow the cookies to cool on a wire rack and enjoy!
Store the cookies in an airtight container on the counter for up to 1 week. Store at room temperature, in a cool dry place.