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

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 over-mix. Place in the refrigerator or freezer until you need it in step 6. Cold crumbs hold their shape in the oven.

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 light and creamy, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Scrape down the sides and up the bottom of the bowl with a silicone spatula as needed. Add the eggs, sour cream, and the vanilla. Beat on medium-high speed until combined. The mixture will 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-coated apples. Top with crumb topping. Using a silicone 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. Baking times vary, so keep an eye on yours. The cake is done when a toothpick inserted in the center comes out clean. If you find the top or edges of the cake is/are browning too quickly in the oven, loosely cover it with aluminum foil.

Remove the cake from the oven and set on a cooling rack. Allow to slightly cool for at least 30-45 minutes before slicing and serving.

Drizzle with optional icing, if desired.

Cover leftover cake tightly and store in the refrigerator for 5 days.