Muddle the mango and strawberries thoroughly with the lime juice and the Demerara Simple Syrup in the bottom of a cocktail shaker. Add the rum and stir. Fill with shaved ice and shake vigorously. Strain into a demerara-rimmed cocktail glass, forming a foamy layer of finely crystallized ice to finish.
Bring 1 cup of water to a simmer, add the sugar and stir to dissolve. Remove from the heat, cover and chill.