Fill a cocktail shaker halfway with ice. Add the tequila, liqueur, lime juice, Simple Syrup and egg whites. Shake for 8 to 10 seconds. Strain into a rocks glass filled with ice.
Put the sugar and water in a small saucepan. Cook over low heat, stirring, until the sugar dissolves. (Alternatively, heat in the microwave.) Store in a sealed container in the refrigerator.