Preheat the oven to 350℉. Line 2 large baking trays with parchment paper or silpat baking mats.
Add the sugar and butter to a large bowl. Use a handheld electric mixer to beat until combined. Beat in the egg and vanilla. Add the flour, salt, and baking powder, and beat until smooth.
Divide the dough into 4 equal parts and roll each into a ball. Place the dough balls on the prepared baking trays, 2 balls per tray. Flatten each ball of dough out to a circle about 4 1/2 to 5 inches in diameter. Arrange 2 tablespoons of M&M candies on top of each cookie, lightly pressing them in.
Bake until the cookies are golden along the outside, but still look just a touch doughy in the center, about 12 to 14 minutes, rotating the trays once halfway through. Let the cookies cool on the trays for about 10 minutes before removing.
Notes
Make sure you don’t crowd your cookies because these get huge. Two cookies per half sheet pan works well.
Make sure your butter is slightly but not fully melted.
I like to remove these cookies from the oven when they look just a touch doughy in the center because they’ll continue cooking on the trays even when they’re out of the oven.