In a shallow dish large enough to fit a slice of bread, whisk together the heavy cream, whole milk, egg yolks, 1/4 cup of granulated sugar, Grand Marnier (if using), vanilla extract, and salt until fully combined and smooth.

Dip each slice of brioche bread into the custard mixture, allowing it to soak for about 30-45 seconds per side. The bread should be saturated but not falling apart.

Melt 1 tablespoon of butter in a large non-stick skillet over medium heat. Place 2-4 slices of the soaked bread in the skillet, ensuring not to overcrowd it. Cook for 3-4 minutes per side, until golden brown and the center is cooked through. Repeat with the remaining butter and bread slices.

Place the cooked French toast on a heat-proof surface or baking sheet. Sprinkle about 1 to 1.5 teaspoons of the remaining 1/3 cup of sugar evenly over the top of each slice.

Using a kitchen torch, carefully heat the sugar until it melts, bubbles, and turns a deep amber color, forming a hard crust. Alternatively, place the baking sheet under a preheated broiler for 1-2 minutes (watch very closely to prevent burning). Let it cool for a minute before serving.