Preheat the oven to 350 F. In a medium bowl, whisk together flour, cocoa powder, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy. Add in egg and vanilla extract and mix until it is smooth.
Add in the flour mixture, and mix until it is fully combined. Roll the cookie dough into balls and place onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
Remove from the oven and top with candy eyes, pretzels and M&Ms.