Using a slice of grapefruit, slide the slice around the rim of the glass and dip the rim in kosher salt. Another alternative to using the grapefruit, is to use a small amount of simple syrup* on a towel and use the towel spread it around the rim of the glass, then dip in kosher salt.
In the jar of a blender combine the grapefruit, ice cubes, tequila, light simple syrup, and lime juice. Blend on high for 1-2 minutes until completely smooth.
Optional Step: strain through a fine mesh sieve to remove pulp, if desired.
Divide evenly into 2 glasses, let sit for 2 minutes and stir before serving.