Warm the cream cheese in the microwave for 15 to 20 seconds until softened. Whisk together the cream cheese, maple, and vanilla until smooth.
Divide the cream cheese mixture and spread it onto 4 slices of brioche or challah, leaving a small border around the outside. Top with another slice of bread and press down gently.
In a wide shallow bowl or container, whisk together the eggs, milk, sugar, vanilla, cinnamon, nutmeg and kosher salt.
Preheat a griddle over medium heat and melt 1 tablespoon butter. Soak each side of the sandwiches 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 (or toss both sides in cinnamon sugar, following the instructions in Cinnamon French Toast).