Set the oven rack to the middle position. Preheat to 300ºF (149ºC). Place the bread on a wire rack set inside of a baking sheet.
Toast the bread for 14 minutes, flipping halfway through cook time. It should be dry to the touch but not browned. After removing from the oven, the bread will become drier as it cools at room temperature.
In a large shallow bowl thoroughly whisk together the milk, eggs, egg yolks, brown sugar, vanilla, cinnamon, nutmeg, and salt.
Soak each slice of bread for 15 to 20 seconds on each side. Use a spatula to transfer to a separate baking sheet. Whisk the custard as needed in between batches to keep the ingredients mixed. Sprinkle more cinnamon on top if desired.
Heat a 12-inch nonstick pan or cast iron skillet over medium-low heat. Melt ½ tablespoon of butter in the pan until it bubbles.
Add two pieces of bread at a time. Cook until the first side is golden brown, dry to the touch, and lightly crisp, about 3 to 5 minutes. Flip the bread over and cook the other side until golden, about 3 to 4 minutes.
Wipe out the pan and add fresh butter, repeat the cooking process with the remaining dipped bread.
Right before serving sprinkle powdered sugar on top of the warm French toast. Serve with maple syrup.
Serving Size: 1 bread slice
Keeping the french toast warm: If making a big batch of french toast, place the fried pieces inside a 200ºF (93ºC) oven. This keeps them warm as the other pieces are being cooked and until ready to serve.
Drying bread in a toaster: Toast each bread slice on the lowest setting. Repeat as needed until the surface is dry but not browned.