In a medium bowl, whisk together 2½ cups (350 g) gluten-free flour blend, 1 teaspoon baking soda, and 1 teaspoon salt until combined. Set aside.
In a large bowl, beat ¾ cup (170 g) unsalted butter and 1½ cups (300 g) granulated sugar with an electric mixer on medium speed for 2 to 3 minutes, until the mixture is pale, light, and fluffy. Scrape down the sides of the bowl as needed.
Add 2 (100 g) large eggs and 1 tablespoon (15 g) vanilla extract to the butter mixture and mix until smooth, about 1 minute.
Add half the flour mixture to the wet ingredients and mix on low until just combined. Add the remaining flour and mix until no streaks of flour remain. Do not overmix.
Gently fold in 1 cup of the 1½ cups (320 g) M&Ms with a spatula, reserving the remaining ½ cup for topping. The dough will be thick.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour. Do not skip this step. The dough needs to chill so the cookies hold their shape and don't spread flat.
When ready to bake, preheat the oven to 350°F and line a baking sheet with parchment paper.
Scoop the dough into 2 to 3 tablespoon balls and place them 3 to 4 inches apart on the prepared baking sheet. You should get about 24 cookies.
Press 3 to 4 of the reserved ½ cup M&Ms into the top of each dough ball so they're visible after baking.
Bake on the middle rack at 350℉ for 11-13 minutes, until the edges are lightly golden and the centers look just set. They will look slightly underdone in the middle but will firm up as they cool.
Let the cookies cool on the baking sheet for 4-5 minutes before transferring to a wire rack. If baking in batches, keep the remaining dough in the refrigerator between batches.