In a large saucepan, melt butter on low heat. Add marshmallows (except for a handful), stirring continually on low heat until entirely melted.
Remove from heat, add in rice krispies cereal and hot chocolate mix until well combined.
Line an 8x8 baking dish with parchment paper and press mixture into the dish while still warm. Sprinkle with the rest of the mini marshmallows and allow to cool for 5 minutes.
Notes
A quick tip when it comes to making Rice Krispie treats - if you press the cereal mixture into the pan too hard, you'll end up with tough, dense bars. Press them enough to even them out in the pan, that's all you need!
You can use regular sized marshmallows, they will just take longer to melt. Try quartering the regular sized marshmallows to help them melt quicker.
If you don’t have hot chocolate mix on hand you can substitute it with the same amount of unsweetened cocoa powder.
Store your hot chocolate rice krispie treats in an air-tight container kept at room temperature for 3-5 days. Be sure to seal them so they don't harden and become stale.