Preheat the oven to 350. Line 2 large cookie sheets with parchment paper.
In a large bowl stir together the flour, sugars, cocoa powder, baking soda, and salt.
Add the egg, vanilla, and butter. Mix until smooth.
Scoop 18 cookies.
Bake for 10-12 minutes, until barely set. Press down the cookies gently to flatten the tops.
Topping
Combine the chocolate chips and heavy cream in a small microwave safe bowl. Heat for 30 seconds. Stir. Heat for additional 15 second increments until the chocolate is 75% melted. At that point stir until the rest of the chocolate melts. Stir in the peppermint extract. Let cool to room temperature.
Top the cookies with the ganache. Sprinkle with the crushed candies and marshmallows.