Prepare your lemon and flower garnish and set aside.
Add the gin and elderflower liqueur to your cocktail shaker.
Add the lemon juice, simple syrup and egg white to your shaker.
Fill the shaker with ice and shake well, about 15-20 hard shakes.
Strain the liquid out of the shaker and discard the ice.
Shake the drink again without ice. You'll want to shake for a while, about 45 seconds to a minute, to create a strong foam. (Instead of shaking, you can also use a milk frother for this step.)
Strain the drink into your serving glass.
Place one end of your twist-handled bar spoon into the drink and slowly and carefully pour the seltzer directly onto the handle of the spoon. The seltzer will trickle down the handle without spilling. (You may need to practice a few times to get this right!)
When the foam climbs above the top of the serving glass, stop pouring the seltzer and remove the bar spoon from the glass.