Note: this dough requires chilling!
Stir melted butter with granulated and brown sugar, then stir in cocoa powder. You can do this by hand or with an electric mixer.
Stir in egg, vanilla, salt, and baking soda. Carefully stir in flour.
Line a cookie sheet with parchment paper or a silicone baking mat. Scoop 1-tablespoon balls of dough and place on baking sheet - spacing doesn't matter because you're going to chill them. Chill dough at least 30 minutes.
Preheat oven to 350°F. Once oven is preheated, spread cookies out to 2-inches apart and bake 8-12 minutes, or until no longer glossy on top and dried looking around the edges. They should flatten. If they've been chilled longer than 2 hours (or if you're baking from frozen) you might want to lightly press them with your palm to get them started spreading before going into the oven.
Immediately after they come out of the oven, press a peppermint patty on top. The patty will melt and then re-harden. Wait for that before you make the ganache.
Heat cream in the microwave or in a small saucepan until just before it boils. Pour over chocolate chips in a medium heat proof bowl. Let sit about 30 seconds, then whisk until the chocolate is all melted and smooth. Spoon a bit of ganache over the top of each cookie, spreading to cover the peppermint patty.
Let sit for a few hours at room temperature to set. Once ganache is set, store in an airtight container. Wait until it's fully set before stacking. You can also freeze these cookies.