Put a coupe or small wine glass in the freezer for 10 minutes, or fill it with ice water while you work. If your açai purée is frozen, thaw it in a bowl of warm tap water for 5 minutes until it squeezes out like thick yogurt. Stir the thawed purée before measuring. It separates slightly, and the thicker part carries most of the flavor.

Squeeze the lemon and strain out the seeds and pulp, which can clump in the foam. Measure the vodka, lemon juice, syrup and purée into a cocktail shaker. Separate the egg white by cracking the egg and passing the yolk between the shell halves over a small bowl. Add the white to the shaker. Even a speck of yolk will weaken the foam, so start over if any gets in.

Seal the shaker with no ice and shake hard for 15 seconds. You should feel the liquid thicken and hear it change from a sloshing sound to a muffled one. This step, called a dry shake, whips air into the egg white before cold temperatures can tighten it. Skipping it gives you a thin, bubbly foam instead of a creamy one.

Open the shaker, add 1 cup of ice and reseal. Shake vigorously for 12 to 15 seconds, until the outside is frosty and too cold to hold comfortably. Don't go much longer. Over-dilution makes the drink watery and the color dull.

Discard the ice from your glass and pour the drink through both the shaker strainer and a fine mesh strainer. This catches the ice shards and any seeds from the açai, so the foam stays smooth. Let it settle for 30 seconds. The foam should rise into a pale lilac cap about half an inch thick. Add the bitters on top if using, then garnish with the blackberries and the lemon peel, twisted over the glass to release the oils. Rebecca's Tip: Taste your lemons before you mix. Winter lemons are often juicier and slightly sweeter than summer ones, and that changes the balance. If the drink tastes sharp, add a quarter ounce more syrup. If it tastes flat, add a quarter ounce more lemon. Adjust by small amounts, one tweak at a time.