In a medium bowl, whisk together the flour, baking soda, and salt. Set this aside.
In the bowl of a stand mixer fitted with the paddle attachment (or in a large bowl using a hand mixer), beat the softened butter, brown sugar, and granulated sugar together on low to medium speed until fully incorporated and fluffy. This usually takes about 1–2 minutes.
Scrape down the sides of the bowl, then add the eggs and vanilla extract. Mix for another 30 seconds, or until the eggs are fully blended into the dough.
Gradually add the dry ingredients into the wet mixture. Mix on low speed just until the dough starts to come together and no streaks of flour remain.
Gently stir in ¾ cup of M&M’s. Save the rest for topping the cookies before baking.
Cover the dough and refrigerate for at least 30 minutes, or up to 3 days. This step helps prevent spreading and enhances flavor.
When ready to bake, preheat your oven to 350°F. Line baking sheets with parchment paper or silicone baking mats.
Roll dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. Press a few extra M&M’s into the tops for that bakery-style look.
Bake for 8–10 minutes, or until the edges are lightly golden. The centers may look slightly underbaked, that’s perfect.
Remove from the oven and let the cookies sit on the tray for a few minutes before transferring to a wire rack to cool completely.