This unique recipe for ice cream in a bag is from Saul Griffith's "Howtoons: The Possibilities Are Endless!"
In a large bowl, whisk together heavy cream, milk, sugar, and vanilla until well combined. Pour mixture into a 1-quart resealable plastic bag. Squeeze out all the air and seal to close.
In another large bowl mix together ice and salt. Fill a 1-gallon resealable plastic bag 1/3 full with ice mixture. Add bag with cream mixture and continue adding ice and salt mixture until bag is filled; you may not need to use all the ice and salt. Seal to close.
Shake bag about 10 minutes until cream mixture is no longer liquid. Let cream mixture rest, covered with ice, another 5 minutes until firm. Serve immediately.