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)