Place the whiskey and the blackberries in a mixing glass and muddle until the blackberries break down - about a minute or two.
Add fresh squeezed lemon juice, simple syrup and orange liqueur to mixing glass and stir.
Place ice in cocktail glass and pour cocktail through a fine mesh strainer into glass.
Garnish with extra blackberries if desired.
Notes
Use fresh squeezed lemon juice. Use your favorite bourbon or whiskey. To make simple syrup - over medium heat, combine equal parts water and sugar. Stir until sugar dissolves. Remove from heat and let cool to room temperature. Store in a lidded glass bottle in the refrigerator for up to a month.