Preheat the oven to 350F and line a rimmed baking sheet with parchment paper. Sprinkle the brownie mix over the pan and bake it for 12-15 minutes, or until the temperature reaches 165F on an instant read thermometer. Set it aside to cool completely. 

Add the cooled brownie mix and melted butter to a large bowl (or the bowl of a stand mixer fitted with the paddle attachment). Slowly drizzle in the half-and-half while mixing. Add just enough half-and-half so the batter comes together. 

Next add the chopped chocolate and mix to combine. 

Serve immediately or store in the fridge until ready to use. If the batter has been in the fridge, it will be hard and stiff– just allow it to come to room temperature before enjoying.