1-1/2 cups well-shaken buttermilk
4 large eggs
3 Tbsp sugar
1/4 tsp salt
12 (1/2-inch-thick) slices challah (from a 1-pound loaf; not end slices)
4-1/2 Tbsp unsalted butter, divided
Whisk together buttermilk, eggs, sugar, and salt in a bowl. Pour into a large 4-sided sheet pan, then add bread in 1 layer and soak, turning occasionally, until bread has absorbed all liquid but is not falling apart, about 20 minutes.
Heat 1-1/2 tablespoon butter in a 12-inch nonstick skillet over medium-high heat until foam subsides. Transfer 4 bread slices with a slotted spatula to skillet and cook, turning once, until slightly puffed and golden brown, about 3 minutes total. Transfer to a large shallow baking pan and keep warm in oven. Cook remaining bread in 2 batches, adding 1 1/2 tablespoon butter between batches.