Step 1

Put the blackberries, sugar, and water in a small saucepan over medium heat. Stir to combine.

Step 2

Cook, stirring occasionally, for 15-20 minutes or until blackberries have softened and syrup has slightly thickened (it should be thicker than water, but a little bit thinner than maple syrup).

Step 3

Strain syrup through a mesh sieve to separate any seeds or lumps from the syrup. You can discard or compost the leftover berry pulp, but we like to freeze it in ice cube trays and pop a cube or two into smoothies.

Step 4

Let syrup cool on the counter for 20 minutes before you make the drinks, or let it cool completely and store in an airtight container in the fridge for up to a week.

Step 5

Combine whiskey, lemon juice, blackberry syrup, and a sprig of fresh rosemary in a large cocktail shaker. Add a handful or two of ice cubes. Depending on the size of your shaker, you may have to shake the drinks in two batches.

Step 6

Shake for 15-30 seconds to chill drinks and ensure all ingredients are combined.

Step 7

Strain drinks into cocktail glasses filled with fresh ice. Top each drink with a splash of tonic water, garnish with a sprig of rosemary (optional), and serve immediately.