Combine bourbon, blueberry syrup, lemon juice, egg white, 2 torn sage leaves, and optional bitters in a shaking tin and add ice. Shake 10-12 seconds, then strain into a chilled mixing glass or shaking tin. Use a hand-held latte whisk on the mixture for 20 seconds, then pour into your chilled serving glass. Garnish with a sage leaf. If you don’t have a latte whisk, dry shake with the egg and 1-2 cubes for 30 seconds, then add ice and shake for another 20-30 seconds. Strain into the chilled coupe glass.