A "sorbet course" before the cake provides an icy, ultralight transition to much richer delights. Our sorbet is made with poached peaches and blush "off dry" (slightly sweet) Champagne.
Put peaches into a large saucepan. Add sugar and enough Champagne to come one-third of the way up sides of the peaches (about 1 1/2 cups). Bring to a simmer over medium heat. Cook until peaches are soft, 10 to 14 minutes. Remove from heat. Pour peaches and liquid through a fine sieve into a medium bowl, and reserve liquid; let peaches stand until cool enough to handle.
Remove skin from peaches, and cut flesh into chunks; discard skin and pits. Puree peaches in a food processor with reserved cooking liquid. Stir in remaining Champagne, the corn syrup and lemon juice. Refrigerate until cold, about 15 minutes. Transfer mixture to an ice cream maker, and freeze according to manufacturer's instructions. Transfer to an airtight container, and freeze until firm, about 2 hours.