Place fresh blackberries and mint leaves at the bottom of a sturdy highball glass. Using a muddler (or the back of a wooden spoon), gently press until berries burst and mint releases its aroma—about 6–8 presses. Don’t pulverize; light muddling avoids bitter greens.

Pour in the simple syrup or agave nectar and lime juice. Give it a quick stir—the mixture should turn a rich purple with flecks of mint.

Measure out 2 ounces of white rum and add it straight to the glass. If you love a stronger cocktail recipe, bump up to 2½ ounces.

Pack in crushed or cubed ice almost to the brim. This chills your cocktail fast and helps with dilution control.

Slowly pour club soda or sparkling water down the side of the glass. As the bubbles rise, they’ll swirl up more berry and mint flavor.

Stir gently once more, then garnish with extra berries, a lime wedge, and a sprig of mint. Taste—add more syrup if you like it sweeter, or another squeeze of lime for tang.