Lightly coat a 9×13 baking dish with nonstick spray. Place bread slices evenly into the baking dish.
In a large glass measuring cup or another bowl, whisk together milk, eggs, vanilla bean paste, cinnamon and nutmeg. Pour mixture evenly over the bread slices. Cover and place in the refrigerator for at least 2 hours or overnight.
Preheat oven to 350 degrees F. Place the bread mixture into the oven and bake for 30-35 minutes, or until golden brown.
Melt butter in a large skillet over medium heat. Stir in brown sugar and maple syrup until well combined and slightly thickened, about 2 minutes. Pour mixture evenly over the bread slices.
Serve immediately, sprinkled with confectioners’ sugar, if desired.