Preheat oven to 350 degrees. Grease a 9"'x13" baking pan. Set aside
In a large mixing bowl, cream together butter, sugar, and brown sugar for about 2-3 minutes. Beat in eggs, one at a time, and add vanilla. Mix until well combined.
In a medium bowl, whisk together flour, baking powder and salt. Alternate adding flour and milk to egg mixture. Stir until combined. Set aside
Apple Mixture
In a large bowl, combine apples, brown sugar, melted butter, flour, cinnamon and salt. Evenly coat apples and set aside.
Assemble
In prepared baking pan, spread half of the cake batter. Top with half the apple mixture and repeat. Use a butter knife to gently swirl batter and apples.
Bake for 45-55 minutes until toothpick inserted in center comes out with only a few moist crumbs. Let cake cool to room temperature.
Notes
When making apple mixture, adjust flour and apple ratio depending upon the juiciness of your apples. Mixture should be somewhere between wet and a dry crumble.
Cake can be stored in an air-tight container for 2-3 days.
Cooled cake can be wrapped in plastic and foil and frozen for up to 3 months.