Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.

Cream together the butter, granulated sugar, and brown sugar until light and fluffy.

Beat in eggs one at a time, then stir in the vanilla.

In a separate bowl, whisk together the flour, baking soda, salt, and cocoa powder. Gradually add to the wet ingredients, mixing until just combined.

Stir in chocolate chips. Drop by rounded tablespoons onto prepared baking sheets, leaving about 2 inches between each cookie.

Bake for 9-11 minutes, or until edges are set and centers are still slightly soft. Cool on baking sheets for a few minutes before transferring to a wire rack to cool completely.

Melt black candy melts. Use a toothpick to draw spider legs onto cooled cookies. Add candy eyeballs if desired. Let set completely.