In a medium bowl, whisk together 2 ¼ cups of all-purpose flour, 1 teaspoon of cornstarch, ½ teaspoon of baking soda, and ½ teaspoon of salt. Set aside.

In a stand mixer, combine ¾ cup of melted butter with 1 cup of brown sugar and ½ cup of granulated sugar. Beat on medium speed for about four minutes until light and fluffy.

Add 1 large egg, 1 egg yolk, and 2 teaspoons of vanilla extract. Beat for 20 to 30 seconds until just combined.

Gradually add the dry mixture into the wet ingredients while mixing on low speed until no dry flour remains.

Fold in 1 cup of Halloween M&M’s and ¼ cup of colorful sprinkles using a wooden spoon or rubber spatula.

Cover the dough with plastic wrap and chill in the refrigerator for at least two hours, or overnight for better results.

Preheat your oven to 350° F (175° C) and line a baking sheet with parchment paper.

Using a medium cookie scoop, drop heaping tablespoons of dough onto the prepared baking sheet, spaced about two inches apart.

Bake for about 12 minutes until the edges are golden brown but the centers are slightly soft.

Let the cookies cool on the pan for about one minute, then add candy eyes and extra M&M’s on top before transferring to a wire rack.

Allow the cookies to cool for about five minutes on the wire rack.