Heat your oven to 350°F and prepare two baking sheets by lining them with parchment paper.

In a medium bowl, whisk together the flour, dark cocoa powder, baking powder, baking soda, and salt. Set aside.

In a large mixing bowl, combine the softened butter, brown sugar, and granulated sugar. Beat with an electric mixer for about 2 minutes, or until the mixture is light and fluffy. (A stand mixer with a paddle attachment works as well.)

Add the egg yolks and vanilla, then continue mixing on medium speed for another 2 minutes, until the mixture becomes pale and airy.

Slowly incorporate the dry mixture into the wet ingredients, mixing on low speed until just combined—do not overmix.

Gently fold in the M&Ms and Halloween sprinkles with a rubber spatula until evenly distributed.

Using a 2-tablespoon cookie scoop, portion the dough into 14 balls. Place them on the prepared baking sheets about 2 inches apart. Press a few extra M&Ms and sprinkles on top and around the sides for decoration.

Bake for 10–11 minutes. As soon as the cookies come out of the oven, carefully press candy eyeballs into the tops.

Allow the cookies to cool on the baking sheet for 5 minutes before moving them to a wire rack to cool completely.