Melt butter in a saucepan over low heat. Stir in sugar, Grand Marnier, water, cardamom & salt. Over medium heat, stir until sugar is fully dissolved & the liquid is heated through. Remove from heat. In a small bowl, whisk egg until well beaten. While whisking egg, slowly pour 2 Tbsp. of the hot mixture into bowl with the egg. Then, while whisking the mixture vigorously, slowly pour the warmed egg mixture back into the sauce.