Heat oven to 350℉/177℃.
Spray a 12-inch skillet with cooking spray. Set aside.
In a large bowl, combine apples and brown sugar.
Stir until apples are completely coated in brown sugar.
Pour into prepared pan, spreading evenly. Set aside.
In a medium bowl, whisk flour, granulated sugar, baking powder, vanilla powder, and salt together.
Sprinkle flour mixture evenly over apples.
Place butter slices evenly over the top.
Bake for 65-70 minutes, or until golden brown and bubbly.