Add mint leaves, lime juice, and blackberry syrup to a shaker and muddle for 30 seconds.
Add ice and bourbon, then shake until extremely cold.
Strain and pour into old fashioned glass filled with ice, then top with soda water.
Notes
To make 1 cup of blackberry syrup: In a small pot, bring 2 cups of blackberries, 1 cup of water, 1 tablespoon of monk fruit liquid sweetener, and 2 teaspoons of lime juice to a simmer. Simmer 15 minutes, then allow to cool off the heat for 10 minutes. Pour through a fine mesh strainer, then pour the strained syrup into a jar for cocktails. Store in the fridge up to 1 month.This recipe makes one cocktail with 2.5 net carbs.