Preheat your oven to 350°F and line two baking sheets with parchment paper.

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

In a large bowl, cream together the softened butter, light brown sugar, and granulated sugar until light and fluffy.

Add the egg yolks and vanilla extract to the butter mixture, beating until pale and fluffy.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Gently fold in the Halloween M&M's and jimmies using a rubber spatula.

Using a 2-tablespoon capacity cookie scoop, portion the dough into 17 balls and place them 2 inches apart on the prepared baking sheets.

Press extra M&Ms, candy eyeballs, and sprinkles onto the dough balls.

Bake the cookies for 10-11 minutes, then let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. (Note: Some eyeballs may melt during baking, so you may need to replace them after baking.)