Ice Cups

A frozen shell cast inside a glass, then turned out and filled — an ice-and-salt trick that still works, provided the syrup is dilute enough to freeze and the cold is aggressive enough to freeze it fast.

• The hinge: syrup concentrationSugar depresses the freezing point, so a very heavy confectioner's syrup may refuse to set into a shell in a salt bath. The recipe names no concentration; if a thick syrup only slushes, thinning it with water toward the sweetness of fruit juice is the obvious remedy.
• Ice and salt proportionsThe original assumes you know the ratio. Use about three parts crushed ice to one part coarse rock salt by volume, layered as you pack around the glasses; the bath will run well below freezing, how far below depending on the mix. The "brine at the bottom" is called for deliberately — it improves contact with the glasses and should be left there, not drained.
• Pounded iceIce from a bag of cubes will not pack tightly enough. Crush it small, in a canvas bag or a towel with a mallet, so it beds against the glass with no air gaps. Air is the enemy of the whole operation.
• Glassware and thermal shockFlat champagne glasses (coupes) are what Oscar means, not flutes. Modern thin-blown or tempered glass can crack in a salt bath; sturdy pressed glass, small metal timbale molds or stainless shot cups are safer and conduct cold better. Bring the glasses to refrigerator temperature before packing rather than going from hot dishwater to brine.
• TimingFifteen to twenty minutes is realistic for an eighth-inch shell in a well-packed bath, but check at twelve. Tilt a glass: when the syrup at the center is still liquid and the walls feel solid, pour off the loose syrup at once. Overshoot and the whole cup freezes solid with nothing to empty.
• Turning outDip the outside of the glass in cool water for two or three seconds, no more, and invert onto a chilled plate. Have a sheet pan in the freezer for the finished cups; they soften in seconds at room temperature and cannot be re-set.
• Curaçao ice creamAn orange-liqueur ice, not a flavor sold today. Fold two to three tablespoons of curaçao or triple sec into a quart of softened vanilla ice cream and refreeze hard; the alcohol keeps it soft, so freeze it firmer than you think you need. "Nut ice-cream" in this period could mean burnt almond or pistachio, among others.
• YieldOne dozen glasses gives twelve servings, each holding perhaps two ounces of ice cream. As written the dish is a course, not a bowl of dessert.
• Freezer versionA domestic freezer at 0F will eventually freeze a shell, but slowly and unevenly, giving coarse crystals and a cloudy cup. The salt bath is doing something a freezer cannot: chilling the glass wall directly and fast.