Prepare the bread. Cut the brioche into eight thick slices, approximately ¾ to 1 inch thick.
Make the custard. In a wide, shallow bowl, whisk together the eggs, milk, orange juice, vanilla extract, and cinnamon until smooth and combined.
Heat the skillet. Place a large nonstick skillet over medium-low heat. Add approximately ¾ teaspoon of butter and allow it to melt.
Dip the brioche. Dip each bread slice into the orange custard for approximately 5–10 seconds per side. Allow excess custard to drip off before placing the bread in the skillet.
Cook the French toast. Cook for 2–3 minutes on each side, until golden brown with a fully cooked, tender center.
Repeat with the remaining bread. Transfer the cooked slices to a plate. Add another small portion of butter to the skillet and continue cooking the remaining slices in batches, whisking the custard again as needed.
Serve. Divide the eight slices among four plates and serve immediately while warm. Sprinkle powder sugar on top.
