In a 12-oz. highball glass, muddle mint, simple syrup, wild berries and lime juice.
Fill glass with crushed ice - it is very important to use crushed ice, not cubed - then add rum.
Stir well until the ice is reduced by 1/3, then top with more crushed ice, stirring until the glass begins to frost on the outside.
Spritz with soda water and stir one last time to incorporate.
Garnish with a sprig of fresh mint that has been dusted with powdered sugar and two long straws to avoid tickling your nose with the garnish.