Make the Simple Syrup: In a small saucepan, combine 1/2 cup water, 1/2 cup granulated sugar, and mint leaves. Bring to a simmer for about 3 minutes until the sugar is dissolved. Set aside and let cool, infusing more as needed. Strain and let cool before using.
For one cocktail: Add 1 to 1 1/2 oz. of mint simple syrup, 1 1/2 oz. vodka, and 4 oz. of grapefruit juice to a glass filled with ice. Stir until combined and garnish with mint.
Notes
Store the remaining simple syrup in an airtight container in the fridge for 2-3 weeks.