Grease a 9 x 13 inch pan with butter, then layer the slices of bread overlapping each other slightly.: As you press the bread into the buttered pan, you should feel the slight tack of the butter against the crusts, and the pan will have a faint, glossy sheen. The overlapping gives the slices room to expand while staying snug enough for the custard to weave between them. If the loaf is slightly stale, it will soften as it soaks, producing a pillow like interior. A common mistake here is overcrowding the pan so the custard cannot circulate, which can leave some slices under soaked and others soggy. Make sure the bread sits level, and if any pieces are much larger, tuck them in so the top surface remains even.
In a large bowl, whisk together eggs, milk, half-and-half, sugar, vanilla extract, salt and cinnamon until well combined.: When you whisk, you will notice a pale, slightly frothy surface and the aroma of cinnamon and vanilla will rise, signaling the flavors are marrying. The custard should look smooth, with no streaks of egg white, and have a silky viscosity that will coat the back of a spoon. Whisking just until combined avoids incorporating too much air, which helps the finished texture stay dense and creamy. Over whisking is a frequent error that leads to a more cake like crumb, so stop once everything is uniform and aromatic.
Pour the custard mixture over the bread slices, ensuring all are evenly soaked. Cover the dish with plastic wrap and refrigerate overnight.: As you pour, watch the custard seep into the nooks of the bread, darkening the crumb and softening the crust. Gently press a few slices down so the liquid reaches the center, but do not mash the bread. Covering keeps the aromas locked in and prevents the top from drying out in the fridge. Avoid the temptation to bake immediately; the overnight rest time is what produces the best custardy interior. A typical oversight is not fully submerging some pieces, which can result in uneven texture after baking.
Preheat the oven to 350°F. Remove the plastic wrap and bake the French toast for 45 minutes, or until golden brown and set.: The kitchen will fill with a hint of anticipation as the oven warms and you prepare to bake. Preheating ensures the pan goes into a stable environment so the custard sets slowly rather than cooking too fast on the edges. If the oven is not fully up to temperature, the center may take longer to set while the top over browns. Use an oven thermometer if your oven runs hot or cold to avoid inconsistent results.
Serve hot with a drizzle of maple syrup and a dusting of powdered sugar for added sweetness.: When the casserole is done, the top will be a warm golden brown with a faint sheen from the custard, and you may hear a gentle crackle as the edges crisp. Insert a knife in the center to check for a slight resistance and no liquid custard, the aroma of baked cinnamon and toasted bread will be pronounced. Letting it bake the full time ensures the interior is set yet still tender. A common pitfall is removing it too early, which yields a runny center; if in doubt, add 5 to 10 minutes and monitor closely to avoid over browning.
Serve hot with a drizzle of maple syrup and a dusting of powdered sugar for added sweetness: As you plate the slices, the maple syrup will soak briefly into the top, adding glossy sweetness and a scent that complements the cinnamon. The powdered sugar provides a soft, snowy finish that contrasts with the warm toast. Serve while warm to enjoy the contrast between the crisped top and the custardy interior. Avoid leaving it out too long, as the texture will firm and lose that just baked tenderness.