Preheat the oven to 375 degrees F. Line one large or two standard sheet pans with parchment paper.
In a medium-sized bowl, whisk together the flour, baking powder, and salt.
Using either a stand mixer with the paddle attachment or an electric mixer, cream the butter and sugar together until smooth and fluffy, 2-3 minutes. Add in the egg and vanilla, and mix on medium-high speed until fully combined. Scrape the bowl, all the way to the bottom. Finally, add the flour mixture, mixing until completely combined. Stir in the M&Ms.
Cover the bowl with the cookie dough and place in the refrigerator for 10-15 minutes, until slightly firmed up.
Using a tablespoon-sized cookie scoop, portion out the dough and briefly shape into balls, then place 1 1/2 inches apart on the prepared baking sheets.
Use a flat-bottomed glass to flatten the cookies until they’re around 1 1/2 inches in diameter. Chill the cookie dough for 15 minutes.
Bake for 8 minutes. Remove from the oven, cool for 5 minutes, then to transfer to a cooling rack.
Store in an airtight container at room temperature for up to three days, in the refrigerator for up to one week, or in the freezer for up to 3 months.