Preheat your oven to 350°F. Grease a 9×13 inch pan. You can also line it with parchment paper. This makes lifting the cake easier later. Set the pan aside while you prepare the batter and crumb topping.

In a large bowl, mix flour, brown sugar, granulated sugar, cinnamon and salt. Pour in melted butter. Mix with a fork until large crumbs form. Do not overmix. You want thick, chunky crumbs. Set aside.

In another bowl, combine flour, baking soda, baking powder and salt. Whisk gently to mix everything evenly. This helps the cake rise properly and keeps the texture light and soft.

In a large bowl, beat softened butter and sugar together. Mix until light and fluffy. This takes about three minutes. Creaming adds air to the batter. That makes the cake soft and tender.

Add eggs one at a time. Mix well after each egg. Then add vanilla extract. Scrape down the sides of the bowl. This ensures everything blends evenly and smoothly.

Add half of the dry mixture to the butter mixture. Mix gently. Then add sour cream. Mix again. Add the remaining dry ingredients. Stir just until combined. Do not overmix. The batter should be thick and smooth.

Spread the cake batter evenly into the prepared pan. Smooth the top with a spatula. Now sprinkle the crumb topping generously over the batter. Cover the entire surface. Press crumbs lightly so they stick.

Place the pan in the oven. Bake for 45 minutes. Check with a toothpick. Insert it into the center. If it comes out clean, the cake is ready. The top should look golden brown.

Remove the cake from the oven. Let it cool in the pan for at least 20 minutes. This helps it set properly. Once cooled, slice into squares. Serve warm or at room temperature.