Preheat oven to 150 degrees.
In shallow dish, whisk together eggs, milk, orange juice and zest, sugar and vanilla until smooth and well combined.
Heat large skillet over medium-high heat. Swirl 1 tablespoon butter in pan to melt and coat the surface.
Briefly dip 1 slice bread into egg mixture, then turn over and dip the other side, as well. Promptly transfer soaked bread directly to skillet. Repeat with 3 more slices of bread, or until pan is full but not overcrowded. Cook for 2 minutes, or until bottoms are browned, caramelized and crisp. Turn and cook for 2 minutes, or until set in center. Transfer French toast to baking sheet, placing slices in single layer. Place baking sheet in oven to keep French toast warm while you cook the remaining slices of bread. Add remaining 1 tablespoon butter to hot skillet and repeat above steps with remaining bread.
To assemble the French toast, place 1 slice on a plate. Using a butter knife, spread slice generously with 1 tablespoon Nutella. Place another slice of French toast on top.