How to Make It
Open 1 bottle of Champagne and pour 5 ounces into a flute; sip while you make the jellies. Pour half the remaining Champagne from the open bottle into a large bowl. Sprinkle the gelatin evenly on top; let stand until dissolved.
In a medium saucepan, bring the remaining Champagne from the open bottle just to a simmer. Whisk in the gelatin mixture and cook over low heat until completely dissolved, then transfer to the large bowl. Whisk in the remaining bottle of Champagne and the agave. Let stand for 5 minutes, then skim off any excess foam.
Strain the mixture through a fine sieve into a lightly oiled 9-by-13-inch baking pan. Refrigerate overnight. Using a paring knife, cut into 1-inch cubes. Using a small offset spatula, carefully remove the jellies from the pan. Dip or sprinkle with decorating sugar and serve.