Preheat the oven to 350°F. Line a couple of baking sheets with parchment paper.

In a stand mixer with the paddle attachment, cream together the butter, brown sugar, and granulated sugar on medium speed for about 3 minutes, until the mixture is light and airy. Pause to scrape down the sides, then mix in the vanilla and egg until smooth.

In a separate mixing bowl, whisk the flour, cocoa powder, baking soda, baking powder and salt. Slowly add the dry ingredients to the butter mixture, mixing just until the dough comes together. It should be slightly tacky.

Using a large cookie scoop, fill the baking sheets with dough balls, leaving 2 inches of space between each.

Press a few candy eyeballs and Halloween jimmies into the dough balls. Bake for 12 minutes, or until the cookies are puffed and set around the edges but still soft in the center. Let them cool on the pan for 5 minutes before transferring to a wire rack to cool completely.