Make the lavender syrup: In a small saucepan, combine 1 cup water and 1 cup sugar. Bring to a simmer, stirring until the sugar dissolves.
Remove from heat, add 1–2 tablespoons culinary dried lavender, and steep 15–20 minutes. Strain and cool. Store in the fridge.
Chill your glass: Place a Collins or highball glass in the freezer for 5–10 minutes, or fill it with ice water while you prep.
Set up your shaker: Add 2 ounces gin, 3/4 ounce fresh lemon juice, 1/2–3/4 ounce lavender syrup (to taste), and 1 egg white or 1/2 ounce aquafaba to a cocktail shaker.
Dry shake: Seal and shake without ice for 10–15 seconds to build a silky foam.
Wet shake: Add ice and shake again for 10–12 seconds to chill and dilute.
Strain: Discard any ice from your glass if you used it to chill.
Strain the mixture into the chilled glass.
Top with soda: Gently add 2–3 ounces chilled soda water. Pour slowly down the side of the glass to preserve the foam.
Garnish and serve: Add a lemon twist or a small lavender sprig. Serve immediately while it’s cold and bubbly.