Preheat oven to 250°. Lightly beat eggs, cream, milk, sugar, cinnamon, vanilla and salt in a large shallow baking dish (a lasagna pan is perfect). Add bread, turn to coat, then press down gently on bread until you feel it start to soak up custard mixture.
Flip bread and soak on second side, pressing down gently from time to time, until bread is saturated but not soggy.
Heat butter in a large skillet over medium heat. When foaming subsides, carefully lift 3 slices of bread from custard, letting excess drip back into dish, and cook in skillet until golden brown and center of toast springs back when pressed, about 2 – 3 minutes per side. Transfer toast to a wire rack set inside a rimmed baking sheet and keep warm in oven while you cook remaining slices of bread with more butter.
Serve French toast with butter, maple syrup or powdered sugar.