Preheat oven to 350°F. Lightly coat a 9x13 baking dish with cooking spray. Set aside.
In a large bowl, combine apples, brown sugar, lemon juice, cinnamon, nutmeg, and vanilla. Stir well to combine, ensuring that all apples are coated with mixture. Spread in an even layer in prepared dish. Set aside.
In a medium bowl, whisk flour, sugar, baking powder, cinnamon, and salt. Add vanilla, milk, and unsalted butter. Stir well to combine. Pour batter evenly over apple mixture. (*SEE NOTE BELOW*)
Bake for 40-45 minutes, or until edges are set and topping is cooked through and golden brown.
Remove from oven and let cool for 15 minutes. Before serving, top with salted caramel sauce and vanilla ice cream, if desired.