2 oz. gin
1 oz. heavy cream
1 egg white
1/2 oz. lemon juice
1/2 oz. lime juice
2 tsp. superfine sugar
2 to 3 drops orange flower water

Combine in a chilled cocktail shaker with plenty of cracked ice, then shake for at least one minute and strain into a chilled Collins glass. Top up with cold seltzer or club soda.