In a wide shallow bowl or container, whisk together eggs, milk, sugar, vanilla, cinnamon, nutmeg and kosher salt.
Preheat a griddle over medium heat and melt 1 tablespoon butter. Soak each side of 4 slices of bread in the egg mixture until it becomes saturated, about 15 seconds per side. Place the bread on the hot skillet and cook 2 to 3 minutes per side, until browned.
Repeat with the remaining butter and 4 slices bread. Serve immediately with maple syrup and a dusting of powdered sugar.