Grand Marnier Syrup:
Combine 4 cups water and 2 cups organic granulated sugar in a sauce pan and bring to a boil. Add orange peel from 2 oranges sliced into thin strips (white pith removed) and simmer until liquid is reduced to 3 cups or thickens to the consistency of syrup. Add 1 Tablespoon Gran Marnier liqueur and cook for 10 minutes longer. Remove orange peel and reserve for garnish on French toast if desired.
In a large bowl, combine all ingredients except sliced Challah bread. Whisk until well blended.
Dip each slice into the egg mixture and then layer in a large glass baking dish. Pour any remaining egg mixture over the bread slices. Cover and refrigerate overnight or at least 8 hours.
Heat a griddle or large skillet over medium-high heat. Melt small amounts of butter into the skillet/griddle. Lightly brown the toast slices on both sides; keep warm in oven until ready to serve. Dust with confectioner’s sugar and garnish with fresh whipped cream and an orange slice before serving. Serve with warm Grand Marnier syrup.