Heat the butter and orange zest in a skillet or chafing dish over medium heat.
Add the orange juice and sugar and whisk until the sugar is completely dissolved and the sauce is bubbling, 2 to 3 minutes.
Stir in the Grand Marnier, if using, and simmer for another minute.
Using tongs, dip a crepe into the orange sauce, then flip it over to cover both sides with sauce. Fold it into quarters and place it onto a warmed serving platter.
Repeat with the remaining crepes and pour any remaining sauce over the crepes on the platter. Serve immediately.
