Serendipity 3 in New York City is its iconic Frrrrozen hot chocolate. Decadent, over the top, it seems to be a contradiction in terms. Indeed, the idea of serving chocolate at ice cold temperature is intuitively problematic, as the cocoa butter will become waxy chips that lack that unctuous mouthfeel we associate with good chocolate. As I experimented with doing it at home, turns out the key is to use cheap instant hot chocolate - the kind that has emulsifiers to rapidly disperse the chocolate in hot water - and supplementing it with good chocolate for the flavor. I personally prefer using really good bittersweet chocolate bars, but using good cocoa powder also works, albeit with a smoother consistency. Commercial chocolate chips are often adulterated with different fats for good baking, but I have had problems with them in a cold preparation. The rest is just a matter of blending with ice and milk.
So, the basic procedure: melt the chocolate, if using, in a microwave. Mix in some of the hot chocolate mix, and enough hot water to make a thick liquid, like a chocolate syrup. In the blender, put in the crushed ice, and milk (I use full fat milk, but could experiment with coconut or almond milk, too). Pour in the chocolate syrup and blend immediately. Pour into serving glasses right away, and use a draw to drink from the bottom up. Whipped cream gilds the lily.