Preheat your oven to 325°F (160°C). Line an 8×8-inch baking dish with aluminum foil, leaving extra hanging over the edges for easy removal later. Spray with nonstick cooking spray to avoid any sticking. Set aside.

In a medium saucepan over medium heat, melt the 3 tablespoons of unsalted butter. Add the diced apples, brown sugar, cinnamon, and kosher salt. Cook for 5-8 minutes or until the apples soften slightly but remain firm. Set aside to cool.

In a medium bowl, combine the melted unsalted butter, both sugars, cinnamon, and salt. Stir until well mixed, then gradually add in the flour, combining thoroughly until smooth.

In a stand mixer bowl, combine cake flour, granulated sugar, cinnamon, kosher salt, and baking soda. Add butter pieces one at a time, mixing until there are no visible butter pieces remaining. Then, pour in the buttermilk, egg, egg yolk, and vanilla extract. Mix until light and fluffy, about 2-3 minutes.

Pour the cake batter into the prepared pan, spreading it out evenly with a spatula. Spoon over the cooked apple mixture, spreading it out evenly over the batter.

Create your crumb topping by breaking up the remaining unsalted butter pieces and spreading them evenly over the apple layer.

Bake in the preheated oven for 55-60 minutes, or until the crumb topping turns golden brown and a toothpick inserted into the cake comes out with a few moist crumbs attached. Keep an eye after 50 minutes.

Allow the cake to cool in the pan for at least 30 minutes. When cool enough to handle, lift the foil out of the pan. Drizzle with caramel topping before serving, preferably while still slightly warm for that heavenly gooeyness!