Cut a notch in a lime wedge, then run the lime around the rim of a glass. Dip the edge of the rim into a plate of flaky sea salt (or for a festive look, use our Margarita Salt).
Place tequila, Cointreau and lime juice in a mixing glass and fill with ice. Stir until cold.
Strain the margarita into the glass with the salted rim. Then carefully pour the Grand Marnier onto the top of the drink over an inverted spoon (float it!). Serve immediately.