Ice-cream made with liquid nitrogen (LN2) is the smoothest you'll ever taste, because it freezes so fast there is no time for large ice crystals to form. Here's a simple way of making layered ice-creams using LN2.
Warning: LN2 is ridiculously cold (it is at its boiling point of 77 K, or nearly 200 degrees Celsius below zero) and hence is inherently dangerous. Do not try to ingest it. Make sure anyone around LN2 understands its perils (Material Safety Data Sheet). Drunks should be kept well away. Do not attempt to confine it. I strongly recommend using a thick-walled polystyrene container to hold it in while making the ice creams; definitely do NOT use a cheap glass-walled vacuum flask: there is an appreciable probability of it cracking and imploding, which trust me, is utterly indistinguishable from exploding.
Step 1: You will need...
- some sort of sweet treat that can be embedded on a stick. We mostly used mini Mars bars, but jumbo marshmallows and brownies worked well too
- popsicle sticks
- half-and-half cream
- vanilla flavoring (or whatever you happen to like)
- food coloring
- a few liters of liquid nitrogen (obtainable from a supplier like Praxair or Airgas, if you have a suitable container to hold it in)
Step 2: Make ice cream mix
This recipe comes from a Nigella Lawson cookbook, but I don't think there is anything especially unusual about it; it's just a custard-style ice cream. It's pretty good, even after we simplified it.
Whisk 5 egg yolks with 125 g of sugar until thick and creamy. Add to a large saucepan with 500 ml of half-and-half milk/cream, and stir over moderate heat for about 10 minutes, stirring constantly. Remove from heat and whisk again. Add flavoring (we used a teaspoon of vanilla essence) and coloring and put in refrigerator until ready to serve.
We made 2 batches of this and divided it into three jars (red, blue and uncolored cream).
Step 3: Make your layered ice cream
Push your mini Mars bar (or marshmallow, or brownie, or cake) onto a popsicle stick. and dunk into liquid nitrogen for a few seconds. Remove, and dunk the cold bar into one of the containers of liquid ice cream mix. Remove, dunk back into liquid nitrogen, and repeat as many times as you have patience for. Make funky patterns and shapes. Don't overdo the LN2 - you just need to set the ice cream, not turn it into a rock. Don't try eating it if it's fuming - it's too cold. Approach it cautiously - you don't want to break a tooth or swallow a deep-frozen piece of ice cream. Once the ice cream is soft, enjoy - it's fantastically smooth.