How to Make It
Prepare the Blackberry Simple Syrup: Bring 4 cups fresh blackberries, 1 cup granulated sugar, and 1/2 cup water to a boil in a saucepan over medium-high, stirring often, until sugar dissolves. Reduce heat to medium-low. Simmer, stirring occasionally, until fruit is soft and liquid is syrupy, 10 minutes. Press mixture through a fine mesh strainer into a jar with a lid, discarding solids. Store in refrigerator up to 1 week.
Process 1 cup blackberries in a blender until smooth. Pour through a fine mesh strainer into a 1-quart jar with a tight-fitting lid, discarding solids.
Add pisco, fresh lime juice, egg whites, and 5 Tbsp. Blackberry Simple Syrup to jar. Cover with lid; shake vigorously 30 seconds or until foamy. Pour mixture into 4 rocks glasses filled with ice. Top each with a dash of bitters, blackberries, and basil.