Preheat oven to 400º and line a baking sheet with parchment paper. Rinse blackberries and roast for 10-15 minutes until they start to break down and lose some of their juices. Remove and set aside.
Fill a rocks glass halfway with Thorntons nugget ice or crushed ice. In a shaker, combine the roasted blackberries, mint leaves, sugar, bourbon and ⅓ cup of ice cubes and shake well. Strain the drink into the glass through a coarse sieve, pressing on the solids.