Make the Slurry: In a small bowl, mix the Cornstarch with 2 tablespoons of cold milk (taken from the 2 cups). Whisk until completely smooth to avoid lumps.
Mix the Base: Pour the remaining milk into a saucepan over medium heat. Whisk in the cocoa powder, sugar, and salt.
Melt the Chocolate: Once the milk is warm (steaming, but not boiling), add the Chocolate Chips. Whisk constantly until fully melted and glossy.
The Thickening Hack: Pour in your cornstarch slurry. Turn the heat down to low and whisk gently for 2-3 minutes.
Watch the Magic: The mixture will suddenly thicken and coat the back of your spoon (like pudding).
Serve: Remove from heat immediately. Pour into mugs and top with whipped cream or marshmallows.