If you'd like a Tajin or salt rim, run a lime wedge around the rim of your glass then dip the rim in Tajin or salt. Set the glass aside until ready to use.
Combine key limes and Himalayan salt in a cocktail shaker or mason jar.
Muddle until juices are released and salt is dissolved.
Add tequila, ice, then shake vigorously for 5 seconds.
Add fresh ice to your prepared glass, then strain the mixture over the fresh ice.
Top with grapefruit soda, stir, and add a lime wheel or wedge for garnish (optional).
Notes
If you can't find grapefruit soda- you can use a mix of sprite and fresh grapefruit juice.
Alternatively- you can mix club soda with fresh grapefruit juice and agave nectar or simple syrup.
Don't skip the key limes & salt- trust me, this really elevates the cocktail!
Stir, do not shake the grapefruit soda! Shaking carbonated drinks will cause your shaker to bust open and make a mess. It also causes the soda to go flat faster.
To make a large pitcher in advance- juice the key limes in a separate glass first, then gently muddle the spent limes with salt before combining with tequila and lime juice. Hold it in the fridge without ice, and don't add the grapefruit soda until you're ready to serve (it will go flat). Serve over fresh ice.