Preheat oven to 450 degrees. Butter bread on one side and arrange in a single layer, buttered side up, on baking sheets. Bake until deep golden, 7 to 10 minutes. Meanwhile, butter a 4-by-8-inch loaf pan and spread jam in an even layer on bottom.
Reduce oven to 350 degrees. Cut toast in half and arrange, cut sides up, in pan. In a medium bowl, whisk together eggs, egg yolk, and sugar.
In a small saucepan, combine milk, cream, vanilla, and salt and heat over medium until just warm; whisk into egg mixture. Slowly pour over toast (being careful not to disturb jam layer); let sit 30 minutes. Bake until toast is deep golden brown and custard is set in middle, 1 hour. (Loosely tent with foil if overbrowning.) Let cool completely on a wire rack, 1 hour. Run a knife around edges before inverting onto a platter.