1 1/2 oz of Calvados (or Apple Brandy)
1/2 oz of Lemon Juice
1/4 oz of Demerara syrup (or simple syrup)
1 dash of absinthe
2 oz of Champagne
Add between a dash and a teaspoon, to taste, of absinthe to Calvados, lemon, and sweetener, shake with ice, and strain into a highball over ice and Champagne (or, more festively, into a Champagne flute over bubbles).