Crema Catalana Foam

Preparation Heat cream and milk with vanilla bean, cinnamon stick, and zests in a 2-quart heavy saucepan over moderate heat until just hot and let stand 5 minutes to infuse. Whisk together yolks, sugar, and flour until thick and pale. Pour hot cream through a sieve into yolk mixture, whisking until smooth, and transfer custard to cleaned pan. Cook over moderately low heat, stirring constantly, until slightly thickened and an instant-read thermometer registers 170°F, about 10 minutes. Pour through a fine sieve into a metal bowl set in an ice-water bath and cool, stirring occasionally. Transfer custard to siphon and screw on top tightly. Unscrew charge holder from head and put in a charger. Reattach charge holder and screw on tightly. Gas will be released into siphon (with a whooshing sound) within a few seconds. Chill filled siphon at least 1 hour to produce a stiffer foam. Remove and discard first charger. (As you unscrew charge holder, you'll hear a brief hissing sound.) If fitting from holder comes out attached to charger, just remove charger and reattach fitting. Screw on second charger and chill at least 15 minutes (longer if foam is not thick enough). Shake siphon well. Turn upside down and press lever to fill 3/4-cup glasses or cups with foam. Serve immediately. Cooks' note:Foam may be made 3 days ahead and kept, chilled, in siphon.

Preparation Heat cream and milk with vanilla bean, cinnamon stick, and zests in a 2-quart heavy saucepan over moderate heat until just hot and let stand 5 minutes to infuse. Whisk together yolks, sugar, and flour until thick and pale. Pour hot cream through a sieve into yolk mixture, whisking until smooth, and transfer custard to cleaned pan. Cook over moderately low heat, stirring constantly, until slightly thickened and an instant-read thermometer registers 170°F, about 10 minutes. Pour through a fine sieve into a metal bowl set in an ice-water bath and cool, stirring occasionally. Transfer custard to siphon and screw on top tightly. Unscrew charge holder from head and put in a charger. Reattach charge holder and screw on tightly. Gas will be released into siphon (with a whooshing sound) within a few seconds. Chill filled siphon at least 1 hour to produce a stiffer foam. Remove and discard first charger. (As you unscrew charge holder, you'll hear a brief hissing sound.) If fitting from holder comes out attached to charger, just remove charger and reattach fitting. Screw on second charger and chill at least 15 minutes (longer if foam is not thick enough). Shake siphon well. Turn upside down and press lever to fill 3/4-cup glasses or cups with foam. Serve immediately. Cooks' note:Foam may be made 3 days ahead and kept, chilled, in siphon.