Fill a shaker halfway with ice. Add gin, bourbon, fresh lime juice, and bitters.
Shake about 10 seconds until the shaker feels frosty.
Strain into a highball glass packed with fresh ice.
Top with chilled ginger beer and give a gentle stir to combine.
Garnish with a lime wheel or mint sprig. Serve immediately.
Notes
Use a spicy ginger beer for the best snap. Vodka is optional and increases strength without changing flavor much. Never shake carbonated ginger beer—add it after straining to preserve fizz.