Lightly grease or spray the inside of your crockpot.
Add the whole milk, heavy cream, and condensed milk to the crockpot. Stir to combine.
Add the chocolate chips and cocoa powder.
Cook on LOW for 2 hours, stirring every 30 minutes to help everything melt evenly.
Once smooth and fully melted, stir in the peppermint extract. Start with 1 teaspoon and adjust based on how strong you want the mint flavor.
Switch the crockpot to KEEP WARM and serve as needed.
Top with whipped cream, marshmallows, or crushed candy canes before serving.