Preheat oven to 325 degrees F and grease an 8-inch baking pan with non-stick cooking spray (I also lined my baking pan to be on the super safe side).
First prepare the rhubarb filling by chopping the rhubarb into 1/2 inch thick pieces. Transfer chopped rhubarb to a large mixing bowl and toss with the sugar, cornstarch and ground ginger. Set aside.
To make the crumbs * whisk the sugars, ground cinnamon, ginger, and salt into the melted butter until fully combined and smooth. With a wooden spoon add the flour. It will feel like solid dough- that's a good thing. Press it together in the bottom of your bowl and set aside.
Prepare the cake batter- In a small bowl whisk together the sour cream, egg, egg yolk, and vanilla extract. Using your mixer fitted with a paddle attachment, mix together the flour, sugar, baking soda, baking powder and salt on low power. Add the softened and cubed butter and one spoonful of the sour cream mixture, increasing speed to medium, mixing just until flour is moistened. Increase speed to medium high for approximately 30 seconds. Add the remaining sour cream mixture in two separate batches, beating for 15 seconds after each addition, and scraping down the sides of the bowl with a rubber spatula.
Scoop a heaping 1/2 cup of batter from the bowl and set aside.
Transfer remaining batter to prepared baking pan, making sure to scrape every last bit out of the bowl. Carefully spoon the rhubarb mixture over the top of the cake batter. Spoon the remaining set-aside batter over the top of the rhubarb; it does not have to be perfectly even.
Use your fingers and break the prepared crumb topping into big, crumby pieces. Try to get the pieces to be approximately equal in size. Sprinkle over the entire cake. Bake for approximately 50-60 minutes or until toothpick inserted comes out clean and cake has set.
Allow your cake to cool completely before serving. Keeps stored in an airtight container in the refrigerator for up to 3-4 days.
This makes A LOT of crumb topping. I like it this way. If the topping isn't your thing, simply reduce amounts by half.