Nutrition per serving may change if servings are adjusted.
2 seedless oranges
1 cup sugar
2/3 cup freshly squeezed orange juice
1/2 cup vodka
1 bottle chilled Champagne, or sparkling wine
To make syrup: With a sharp knife, thinly slice oranges. Place in a medium nonreactive saucepan and toss with sugar. Let stand for 15 minutes.
Add orange juice and vodka to saucepan. Bring to a boil over medium heat. Cover and cook for 5 minutes. Uncover and cook until liquid is syrupy and orange slices are translucent, about 20 minutes.
Strain syrup through a fine sieve into a bowl, pressing on solids to extract as much liquid as possible. Discard solids. Cover and refrigerate until chilled.
To make cocktails: With a vegetable peeler, remove 8 long strips of zest from the orange. Twist each strip and place in a champagne glass. Add 1 tablespoon bitter-orange syrup to each glass. Top with Champagne and stir gently. Serve immediately.
Make Ahead Tip: Cover and refrigerate the syrup (Steps 1-3) for up to 1 week.