Place the blueberries, mint leaves, sugar, and lime juice in a glass. (I recommend a 12-ounce glass or mason jar, but you can obviously use whatever you like.) Using a muddling tool or wooden spoon, muddle the ingredients in the bottom of the glass until the blueberries have released their juices.

Fill the glass with crushed ice and pour the rum over. Give it a stir.

Top with chilled champagne or Prosecco and garnish with extra mint and blueberries if you’re feeling fancy.