Slow Cooker French Toast Casserole

Name: Slow Cooker French Toast Casserole
Ingredients: 1 tablespoon unsalted butter , softened; 16-ounce French brioche loaf, cut into 3/4-inch cubes ( 12 cups ); 8 large eggs; 4 cups half and half; 2 tablespoons granulated sugar; 1 tablespoon brown sugar; 2 teaspoons pure vanilla extract; 1 1/2 teaspoons ground cinnamon; 1/4 teaspoon ground nutmeg; 1/4 teaspoon kosher salt; Powdered sugar , for sprinkling, optional; Pure maple syrup , for topping, optional