Preheat the oven to 325F. Butter an 8x8-inch square baking pan or spray with baking spray. Line with parchment paper.
For the Crumb Topping:
In a large bowl, whisk together ¾ cup flour, sugars, cinnamon, and salt. Using a fork, stir in the melted butter until mostly combined but still crumbly. Stir in the remaining 1/2 cup flour until the mixture is very crumbly and still has large pieces.
For the Cake:
In a bowl, whisk together flour, baking powder, baking soda, and salt.
In a large mixing bowl, combine the butter and sugar and beat on medium speed until light and fluffy, about 3 minutes. (Use the paddle attachment if using a stand mixer.) Add the egg and beat until combined. Scrape down the bowl. Add the sour cream, milk, and vanilla, and beat on medium-low speed until combined. (The mixture will look runny and curdled.)
With the mixer on low speed, slowly add the flour mixture to the butter mixture. Beat just until all of the flour is incorporated, stopping to scrape down the bowl as needed. Transfer the batter to the prepared pan and spread it evenly. (Batter will be very thick.) Sprinkle the crumb topping all over the batter.
Bake for 40 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. If the cake is browning heavily before 40 minutes, loosely cover with foil. Let the cake cool completely in the pan. Sprinkle with confectioners’ sugar before slicing and serving.
Make sure to measure your flour correctly. I highly recommend always using a scale to measure. However, if you don’t have a scale, be sure to fluff your flour with a spoon and then spoon it into your cups before leveling it off with a knife. This method is the best way to measure flour without overpacking the measuring cup.
Do not overmix the crumb topping, or it may become paste-like in texture. You want there to be some crumbly texture to the topping.
Ensure that the baking soda and baking powder are fresh and haven’t expired. One of the main reasons that cakes don’t rise and become dense is because the baking soda and powder have expired.
Bring all the ingredients to room temperature so it mixes quickly and easily.
I recommend tapping the cake pan on your counter to pop any air bubbles before adding the crumb topping. Doing so will help the batter even out in the pan as well.
Make sure your brown sugar is soft and moist for the crumb and cake. If your brown sugar has hardened, don’t worry, it’s an easy fix! I wrote a detailed post on how to soften brown sugar. If you don’t have brown sugar, you can follow this post on homemade brown sugar.