Cover the bottom of an old-fashioned glass or mason jar â€” if you want a more rustic presentation â€” with blueberries. (If you’re using a smaller glass, make a double layer.) Add the brown sugar and lemon juice and muddle (or smoosh with the back of a spoon) until the sugar is melted. The idea is to break the skins of the blueberries, but not to mash them into a pulp. Fill the glass with crushed ice and then add the rum. Top with ginger ale and stir.
Makes one cocktail.