In a medium skillet, set over medium heat, add a tablespoon of butter. When the butter has melted, dip a few slices of the challah bread into the egg nog mixture and then flip them to the opposite side. Transfer to the skillet to cook for abut 2 to 3 minutes on each side, until golden brown. Place the cooked French toast on a sheet pan in a warm oven. Repeat with the remaining slices of challah french toast, adding more butter as needed.