The French 75 is a classic Champagne cocktail that came out of the Prohibition era. Traditionally, it's made with gin (or cognac), lemon juice, and champagne, but you can substitute any kind of spirit for the gin and it'll always work beautifully.
1/2 ounce simple syrup, heat equal parts sugar and water in a pot until sugar dissolves; cool before use
1/2 ounce freshly squeezed lemon juice
1 ounce gin
3 ounces sparkling wine
1 lemon twist
Put simple syrup, lemon juice, and gin in a cocktail shaker. Add ice, seal, and shake. Cover with a Hawthorne strainer, then pour through a fine-mesh strainer into a Champagne flute. Top with sparkling wine. Rub a lemon twist around the rim of the glass, and add it to the drink as garnish.