Preheat oven to 350 degrees and line baking sheets with parchment paper or silicone mats.
Melt the butter and allow to slightly cool (if it’s too hot it may cook the eggs when added).
Prepare dry ingredients by whisking together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a mixing bowl, with an electric mixer, beat the melted butter and sugar until fully combined. Add in the egg and vanilla extract and continue mixing on med-high until well combined. The mixture will start to smooth out and grow in volume.
Add the dry ingredients in three sections, beating on low speed between each addition. After the final addition, stop beating once most of the flour has been incorporated but there is still a small amount visible.
Add the M&M’s and stir into the dough with a rubber spatula. Stop mixing as soon as they are evenly dispersed throughout to avoid overmixing the dough.
Scoop out cookie dough balls onto prepared cookie sheets. Each cookie dough ball should be 30g or 1 heaping tablespoon.
Bake cookies for 10-12 minutes and remove from oven when bottom corners are very, very slightly brown. Allow cookies to settle on the tray for about 5 minutes before transferring cookies from the sheet to a wire cooling rack.