Chef José Andrés uses Sucro, a powdered emulsifier beloved by avant-garde chefs, to make the salty foam topping. It's available at tienda.com. More Margarita Recipes
Fill a cocktail shaker with ice. Add the tequila, Cointreau and lime juice and shake well. Strain into a chilled coupe.
Combine all of the ingredients in a large bowl. Using an immersion blender, mix until bubbles form. Carefully spoon on top of the drink.
I've actually had this drink at his restaurant. I'ts very strange.