Preheat the oven to 325°F convection (with fan) or 350°F conventional (no fan). Spray a 9x13-inch baking dish and line the bottom with parchment paper.

In the microwave, melt the butter for the crumble.

Combine all the ingredients for the streusel topping in a bowl and mix to combine. Set aside.

Peel, core, and dice your apples.

Combine the light brown sugar, cinnamon, and diced apples in a large bowl and set aside. I do not mix this more than 10 minutes ahead of time because sugar draws the water out of the apples, and you will have too much excess liquid. If that happens, don’t discard it, just try to distribute it across the whole dish.

Stir together the kosher salt, baking powder, and all-purpose flour in a medium bowl and set aside.

In the bowl of a stand mixer fitted with the paddle attachment or in a bowl with a hand mixer, cream the butter and sugars together.

Beat in the eggs one at a time until emulsified followed by the vanilla extract. If your eggs are cold, this mixture might break, but don’t worry about it.

In a separate bowl, whisk together the milk, sour cream, and vanilla. Then, with the mixer on low, alternately add the flour mixture and the wet ingredients, starting and ending with flour. I like to finish mixing the batter by hand to ensure minimal gluten development and even incorporation.

Spread about half the batter into the prepared pan and smooth out with an offset spatula. Cover with the apple mixture.

Spread the remaining batter over the top and sprinkle the crumb topping over the entire top.

Bake in the preheated oven for 50-60 minutes or until a cake tester or toothpick comes out with just clinging crumbs. It will puff in the center and be a nice golden brown.

Remove it from the oven and allow to cool for 30-45 minutes on a wire rack before slicing. You can also cool it completely and serve it at room temperature.