Best Gluten-Free French Toast

0 Begin by slicing the bread into thick slices. The bread should not be warm or fresh out of the oven. Place the slices on a large rimmed baking sheet or in a rectangular baking dish, side by side and not overlapping.
1 To make the custard, combine the egg, vanilla, and about half of the milk in a medium-sized measuring cup or bowl. Whisk everything together thoroughly, beating the egg until smooth. Whisk in the 2 tablespoons granulated sugar, and flour until smooth and well combined. Whisk in the remaining milk until smooth.
2 Pour the custard slowly over the brioche slices, allowing the bread to absorb as much as possible. Turn the bread over and spread the remaining custard on the underside. Allow the bread to sit for about 20 minutes, flipping it occasionally. The bread should have absorbed the majority of the custard.
3 As the bread is nearly done soaking up the custard, place one tablespoon of unsalted butter in a skillet with a thick bottom and at least one inch high sides. Place the pan over medium high heat and heat until the butter is completely melted.
4 Place two slices (or as many as will fit without touching) in the pan. Spread a thin layer of granulated sugar on the raw tops of the bread slices. Allow the bread to cook for at least 3 minutes, or until the underside is golden. Carefully flip the slices in the pan and cook for 4 minutes more, sugar side down, or until golden brown on both sides.