Muddle the fruit & mint. Place 3 raspberries, 3 blackberries, 6 mint leaves, 3 dashes cardamom, 2 tsp sugar, and juice of 1 lime in the bottom of your tumbler. Use your muddler to press and twist until the berries break and the mint releases aroma. Don’t pulverize into mush; you want flavor, not a smoothie.
Add rum. Pour 2 oz white rum over the muddled mixture. Stir gently to dissolve the sugar and lift flavors.
Add ice & top. Drop in 3 ice cubes, top with club soda to taste (about 2–3 oz), and add one more dash of cardamom on top.
Garnish & serve. Garnish with a mint sprig and a few fresh berries. Serve with a straw (or not—cute glass photos are optional but encouraged).
Pro tip: Use chilled club soda for maximum fizz. If you prefer a lighter cocktail, reduce rum to 1.5 oz and add a splash more soda.