Combine cinnamon, nutmeg, sugar and vanilla; add to apples and mix well. Spread into a greased 9x13 baking dish.
Using the same bowl, make the topping by combining the flour, sugar, baking powder and salt. Pour over top of the apple mixture and spread it out evenly.
Pour melted butter all over the top of the cake. Tilt the pan if necessary to get the butter to cover as much of the surface of the cake as possible.
Bake at 350° for 45 to 50 minutes until bubbly and lightly browned.
Eat warm or cold as is or serve with milk, whipped cream or ice cream.
Notes
You can adjust the amount of melted butter based on how crispy you prefer the topping to be.