In a medium mixing bowl, stir together the flour, baking soda, cream of tartar and salt. Set the bowl aside.

In a large mixing bowl using a stand mixer or electric hand mixer, beat the butter, brown sugar and granulated sugar at medium speed until light and fluffy. Beat in the eggs and vanilla at low speed until well combined.

On low, gradually add the flour mixture and beat just until combined. Beat in the M&Ms on low.

If making the whole wheat / all-purpose flour version, roll into 1 1/2" balls (about 65 grams) - pushing any M&Ms that stick out back in. If your dough is too soft to roll into proper balls, refrigerate for about 30 minutes or until firm. Freeze dough balls for 2 hours, or until firm.

If making the gluten-free version, freezing is not necessary. Wrap the dough in plastic wrap and refrigerate for at least two hours. Then roll into balls.

Preheat the oven to 350°F for the all-purpose flour version or 375° degrees F for the gluten-free version.

Line a baking sheet with a piece of parchment paper. Place the dough balls 4" apart on the prepared baking sheet. Bake the cookies for 16-20 minutes or until lightly and evenly browned.

Remove from the oven and immediately press 7 more M&Ms on top of each cookie, if desired. Let the cookies cool completely on the baking sheet.

Store in an airtight container for up to 4 days.