Crush the ice. Wrap several ice cubes in a clean towel and tap with a rolling pin or muddler until the ice breaks into crushed pieces. Fill your glass or mule mug with crushed ice.
Add the mezcal and guava juice. Pour 1 1/2 oz mezcal joven and 1 1/2 oz guava juice over the ice.
Top with ginger beer. Add 1 oz chilled ginger beer; give the cocktail a gentle stir with a bar spoon to combine flavors.
Finish with lime. Squeeze a thin lime slice over the drink, drop the slice in, and serve immediately.
Pro tip: Use crushed ice for the best mouthfeel and slower dilution. Want it stronger? Keep the ginger beer as a topper only.