Preheat the oven to 350°F (177°C) and grease a 9×13-inch baking pan or line with parchment paper.

Make the crumb topping: Mix the brown sugar, granulated sugar, cinnamon, and salt together in a medium bowl. Stir in the melted butter. Using a fork, lightly mix in the flour until crumbles form. Don’t overmix.

Make the cake: Whisk the flour, baking soda, baking powder, and salt together. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on high speed until smooth and creamy, about 2 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Add the eggs, sour cream, and the vanilla. Beat on medium-high speed until combined. The mixture may look curdled; that’s ok. Scrape down the sides and up the bottom of the bowl as needed. With the mixer on low speed, mix in the dry ingredients until smooth. Do not overmix. The batter will be thick.

Toss the chopped apples and cinnamon together.

Spread the batter evenly into prepared baking pan. Top evenly with cinnamon apples. Top with crumb topping. Using a rubber spatula or the back of a large spoon, press the crumb topping tightly down into the cake so it sticks.

Bake for 45-55 minutes. The cake is done when a toothpick inserted in the center comes out clean. If browning too quickly, cover loosely with foil.

Cool for 30–45 minutes before slicing and serving.

Top with icing, if desired. Cover leftovers tightly and store in the refrigerator for 5 days.`),