Preheat oven to 300 degrees. Place bread on a wire rack and allow to dry out in the oven for 16 minutes, turning bread halfway through. Remove bread and allow to cool for 5 minutes.
Reduce oven temperature to 200 degrees and return wire rack to the oven.
In a small baking dish or pie plate (large enough for a slice of bread to lay flat), whisk together warm milk, egg yolks, sugar, cinnamon, 2 tablespoons melted butter, salt, and vanilla extract. (The warm milk keeps the melted butter from clumping).
Meanwhile heat ½ tablespoon of butter in a 12-inch skillet over medium-low heat.
Soak bread in milk mixture, about 20 seconds per side, careful not to over saturate the bread so that it falls apart.
Cook bread, 2 slices at a time, in the skillet until golden brown, about 3 minutes. Flip and cook until second side is golden brown, an additional 3 minutes.
Transfer cooked French toast to baking sheet in the oven to keep warm. Wipe skillet clean and repeat the process until all pieces are cooked. Serve with powdered sugar and/or maple syrup.