In a medium bowl whisk 1/4 cup all-purpose flour into 1 cup whole milk until smooth.
Whisk in 4 eggs, 2 tsp vanilla extract, 1 tsp ground cinnamon, 1/2 tsp ground nutmeg, 1/4 tsp salt, and 1 tbsp sugar until combined. Pour the mixture into a shallow pan or wide bowl that will fit a bread slice.
Preheat a large nonstick griddle or heavy skillet over medium heat.
Working one slice at a time, submerge a thick slice of bread in the egg mixture and turn to coat both sides. Let the slice sit in the mixture about 15–30 seconds so the custard is absorbed, then lift and allow excess to drip back into the pan.
Place the soaked bread on the hot griddle and cook until golden brown, about 2–3 minutes per side. Flip and cook the other side until golden. Repeat with remaining slices.
Transfer cooked slices to a plate or wire rack. Spread 1 tablespoon creamy or crunchy peanut butter on each cooked slice (use all 12 tbsp total, one per slice).
Serve slices individually or make sandwiches by pairing two peanut-buttered slices (makes 6 sandwiches). Drizzle with maple syrup and dust with powdered sugar if desired.