Fill a rock glass (also known as an Old Fashioned glass) with crushed ice.
Fill a cocktail shaker with ice and pour in the gin, lemon juice, simple syrup, and blackberry liqueur. Put lid on shaker and shake for 10 seconds.
Strain the mixture from the cocktail shaker into the rock glass.
Garnish with a half slice of lemon and two blackberries.
For a fancier serving option, only shake the first three ingredients and strain into the glass. Then drizzle the Crème de mure over the top of the ice in the glass. Garnish. This is actually how to original Bramble cocktail was served in the 1980's.