Begin by preheating your oven to 350°F (175°C). This ensures that your cookies bake evenly.
In a medium bowl, whisk together the all-purpose flour, baking soda, cornstarch, and kosher salt. Set this mixture aside as you prepare the wet ingredients.
In a large mixing bowl, combine the melted butter, light brown sugar, and granulated sugar. Mix until smooth and creamy. Then, add in the eggs and vanilla extract, whisking until fully incorporated.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Be careful not to overmix, as this can lead to tough cookies.
Fold in the semisweet chocolate chips and 3/4 cup of the M&Ms. This is where the fun begins, as the colors of the M&Ms start to pop in your dough.
Using a cookie scoop, portion out the dough onto the prepared baking sheet, leaving enough space between each cookie to allow for spreading.
Sprinkle the remaining M&Ms over the tops of the cookie dough balls to ensure they look festive and colorful once baked.
Place the baking sheet in the preheated oven and bake for 10-12 minutes, or until the edges are golden brown but the centers remain soft.
Once baked, remove the cookies from the oven and allow them to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. Enjoy your delicious M&M Christmas Cookies with a glass of milk or hot cocoa!