Make the crumble: In a medium bowl, combine the sugar, flour, cinnamon, and salt. Stir in the melted butter. You should have about 1 3/4 cups crumble mixture. Place bowl in the fridge to chill and set while you make the cake batter.
Adjust oven rack to center position. Heat oven to 350 degrees. Line an 8×8 baking pan with parchment paper, leaving enough overhang to use as handles when extracting the cake later. Grease the paper.
Make the batter: In a large bowl, beat the sugar, egg and yolk, sour cream, and vegetable oil until sugar is mostly dissolved and mixture is smooth. Stir in milk and vanilla. Sprinkle flour, baking powder, and baking soda evenly over the top and combine until mixture is fully incorporated. (It’s a fairly thick batter, closer to a quick bread batter than a sponge cake batter.)
Spread a little more than half the batter into the pan. Remove the crumble mixture from the freezer and break it up with your hands. Sprinkle about 2/3 cup of the crumble mixture over layer of batter. Spread remaining batter over the crumbles (don’t be too precious here; it’s okay if some of the crumbles poke through). Cover with the remaining crumble mixture (it will be a thick layer!).
Bake for 25 minutes. Remove pan from oven and, using a wooden spoon, dig into the half-baked cake, pressing the crumble into the partly-set batter and pulling the edges inward. Hack and stir vigorously to create peaks and valleys across the cake. Mixture will deflate a little bit; don’t worry. Return pan to oven. Bake for an additional 20 to 25 minutes, or until a toothpick inserted into the thickest part of the cake comes out mostly clean with no wet batter.
Cool cake in the pan for 20 minutes, then, tugging on the parchment, remove cake to a cutting board. Slice and serve. Cake will keep at room temperature for about 3 days.