In a large bowl, whisk to combine eggs, heavy cream, vanilla bean seeds (or extract, if using), salt and 1/4 cup sugar. Arrange brioche slices in a 9-inch x 13-inch baking dish (or whatever pan all of the slices fit most snuggly in). Pour custard over top and let sit 20 minutes. Carefully flip slices and let them sit another 20 minutes to fully soak up custard.
Meanwhile, preheat oven to 325°F. Line a baking sheet with parchment paper.
In a small bowl, mix remaining 1/4 cup sugar with cinnamon. After the brioche slices have soaked, sprinkle half of the cinnamon sugar mixture evenly over their surface.
Melt 1 tablespoon butter in a large skillet over medium-low heat. Fry three brioche slices, sugar side down, until browned and caramelized, 3- 5 minutes. Sprinkle bare surface with cinnamon sugar mixture, flip the toast and cook another 3- 5 minutes. Transfer to prepared baking sheet. Wipe out skillet (or rinse, if needed, to remove burnt sugar), then melt remaining tablespoon of butter and repeat frying the last three slices of brioche. Bake for 12- 15 minutes, or until fully cooked when a knife is inserted into the center of the bread.