How to Make It
Whisk together eggs, yolks, milk, and cream until combined. Add sugar, liqueur, vanilla, and salt; whisk together until sugar is dissolved.
Place bread slices face down in a single layer in a 10- x- 15-inch baking dish or roasting pan. Pour custard over bread slices. Cover in plastic wrap and refrigerate 8 hours or overnight, flipping the slices over in the custard after 1 hour of soaking.
When french toast has finished soaking, line a rimmed baking sheet with aluminum foil. Preheat oven to 375°F. Combine brown sugar, butter, and corn syrup in a medium saucepan over medium-high. Stir until sugar is dissolved and boiling. Pour sugar mixture onto baking sheet and spread evenly across baking sheet while mixture is still hot. Gently lift each piece of toast out of custard, allowing excess to drip off, and place face down onto the warm caramel.
Transfer baking sheet to oven and bake until the top of the french toast is dry and lightly browned, 25 to 30 minutes. Immediately serve french toast by flipping each piece over so the caramel covered side is facing up. Serve by itself or with whipped cream, if desired.