Preheat the oven to 350°F (180°C). Line two baking sheets with parchment paper and set aside.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large mixing bowl using a hand mixer, beat the softened butter, brown sugar, and granulated sugar until light and creamy. Scrape down the sides as needed.
Mix in the egg and vanilla extract until fully combined and smooth.
Add the dry ingredients and fold in with a large spoon or rubber spatula until combined and a dough forms. Do not overmix.
Fold in the M&Ms and chocolate chips.
Use a 2 tablespoon cookie scoop to scoop dough directly onto the prepared baking sheets, spacing cookies about 2½ inches apart.
Bake for 12–14 minutes, until the edges are set and lightly golden but the centers still look soft.
To get a uniform round shape, immediately after removing from the oven, swirl a round cup or cookie cutter slightly larger than the cookies around each one to create a perfect circular shape.