Preheat the oven to 350℉ (175℃) and add a piece of butter into a 9x13 inch pan; let your pan into the oven while it preheats.
Mix Brown sugar, water, cornstarch, lemon juice, vanilla, cinnamon, nutmeg, and salt in a saucepan. Add the apples and cook over medium heat for 3-5 minutes, stirring.
Pour the mixture into the prepared pan.
Mix the flour, sugar, baking powder, and salt in a large bowl. Add the milk and melted butter and stir until it is combined.
Pour the mixture over the apples in the pan.
Bake for about 38-42 minutes or until a toothpick comes out clean.
Let it cool for at least 15 minutes before serving. You can serve with vanilla ice cream if you'd like.