Preheat your oven to 350 degrees, and spray an 8-inch square pan with baking spray. Line your pan with parchment paper that has over-hanging edges for easy removal and set aside.

In a large bowl, beat together the butter, sugar, sour cream, buttermilk, eggs, flour, baking powder and salt until you have a thick batter. This is an odd mixing method for cake, but it works well here to mix everything together at once, so trust the process!

Spread about half of the cake batter into your prepared cake pan, and set aside while you prepare the filling and crumb topping.

In a small bowl, beat together the cream cheese, sugar, and salt until well combined and smooth. Spread this over the first layer of cake batter evenly, going almost to the edges. Spread the jam evenly over the cream cheese mixture that you have spread into your cake pan. If your jam is very thick, you may want to microwave it for 20 seconds for easier spreading- it shouldn't be hot. Then, spread the remaining cake batter over the top of the filling layer. This doesn't have to be perfect, but it's a bit easier if you dollop the remaining batter all over the top, and the spread with an off-set spatula or the back of a spoon.

In a medium bowl, stir together the sugar, brown sugar, cinnamon, salt, cornstarch, and flour until well combined. Pour in the melted butter, and use a fork to work the mixture together until everything is well combined. The mixture will be dry, but should hold together when you press it in your hands. If it does not hold together in clumps, add another 1-2 tbs. of melted butter. If it's too wet, add another 1-2 tbs. of flour until you reach a crumbly consistency. Crumble the entire mixture over the top of your cake evenly, covering everything.

Bake for 50-55 minutes, or until the cake looks set, and a toothpick inserted into the center of the cake comes out with a few crumbs. You may see streaks from your jam/cream cheese layer as well here. Cool the cake completely in the pan, and slice to serve!