2 fluid ounces gin
1 teaspoon simple syrup
1/2 fluid ounce lemon juice
5 fluid ounces brut champagne, chilled
1 slice lemon (optional)
Chill cocktail shaker and glass (champagne flute or collins glass) in the freezer. Add ice to shaker. Pour gin, simple syrup, and lemon juice into shaker. Shake well. Fill chilled glass half full of ice, then strain cocktail into the glass. Top with champagne and garnish with a lemon slice.