In a large baking dish, whisk eggs, milk, and cinnamon. Set aside. Place the bread slices in the egg mixture. Let soak until the egg mixture is completely absorbed into the bread, turning over occasionally, for 1 to 2 minutes. Heat a griddle over medium low heat. Add the butter and melt. Cook the bread on the griddle until brown and heated through, about 2 to 4 minutes per side. Make 2 sandwiches with the bread and apple pie filling. Cut each sandwich in half. Transfer to plates and serve with additional butter. Top with powered sugar, if desired.