Add the blackberries to an empty cocktail shaker. Use a muddler to crush the berries. Add the cinnamon whiskey, blackberry liqueur, Fernet-Branca, and lemon juice. Top with ice and stir the cocktail for at least 10 to 15 seconds.
To serve, strain the cocktail through a fine sieve into a small coupe or Nick and Nora glass. Garnish with fresh blackberries if desired and enjoy!