If you are rimming your highball glass with salt do this now. Fill the glass 1/2 way with ice.
Rub the basil leaves in clean hands to release the fragrance. Place in a cocktail shaker. Add the orange juice, tequila, Cointreau and lime juice. Muddle together with the back of a spoon.
Fill the cocktail shaker 1/2 way with ice. Shake until well combined. Strain into prepared glass. Add more ice if needed. Garnish with orange slices and a basil leaf.