In a medium bowl, combine the gluten-free flour blend, baking soda, and salt. Whisk together and set aside.
In a larger bowl, cream together the softened, unsalted butter and granulated sugar with an electric mixer until light and fluffy, about 2-3 minutes. Scrape the sides of the bowl as needed.
Mix in the eggs and vanilla extract until smooth.
Gradually add half the flour mixture, stirring until just combined, then add the remaining flour, mixing until no streaks of flour remain.
Gently fold in 1 cup of M&Ms, reserving the rest for later.
Chill the dough for at least one hour.
Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Form the dough into 2-3 tablespoon balls, placing them 3-4 inches apart on the baking sheet. You should get about 24 balls.
Press the remaining 1/2 cup of M&Ms into the tops of the dough balls, using 3-4 for each ball.
Bake on the middle rack at 350°F (175°C) for 11-13 minutes until the edges are lightly browned and the center is set.
Let the cookies cool on the sheet for 4-5 minutes, then transfer to a cooling rack to finish cooling. Repeat with remaining dough. Keep the dough in the refrigerator if you are baking in batches.