Make thyme simple syrup by combining 1 cup water, 1 cup sugar, and 4-5 thyme sprigs in a pot. Heat until sugar dissolves. Cool and strain.
Add blackberries and thyme leaves to a shaker. Muddle gently to release juices.
Pour in gin, lemon juice, and simple syrup. Fill shaker with ice.
Shake vigorously for 10-15 seconds.
Fill rocks glass with crushed ice. Double strain mixture into glass.
Float crème de mûre on top by slowly pouring over the back of a spoon.
Garnish with fresh blackberries and thyme sprig.
Notes
The cocktail tastes best with fresh blackberries during peak season (July-September). Frozen berries can work in a pinch.Store extra thyme simple syrup in an airtight container in the fridge for up to 2 weeks.For a non-alcoholic version, replace gin with club soda and skip the crème de mûre.Strain twice to remove berry seeds and small thyme pieces for a smoother drink.