Prep the rim: Mix equal parts tajín (or chili powder) and kosher salt on a small plate. Rub a lime wedge around the rim of a highball glass, then dip to coat. Fill the glass with ice.
Shake the base: In a shaker, add 2 oz tequila, 2 oz fresh grapefruit juice, 1 oz mango puree, 0.5 oz lime juice, and a small pinch of chili powder.
If your grapefruit is extra tart, add 0.25–0.5 oz agave syrup.
Shake with ice: Fill the shaker with ice and shake for about 10–12 seconds until chilled and lightly frothy.
Strain and top: Strain into your prepared glass over fresh ice. Top with 2–3 oz grapefruit soda (or club soda for less sweetness). Give a gentle stir.
Garnish: Add a lime wheel and a thin slice of mango.
For a little drama, dust a tiny pinch of tajín over the top.
Taste and adjust: If you want more heat, add a dash more chili on top. If you want more sweetness, slip in a touch of agave and stir.