1. Preheat oven to 325 degrees. Spray an 8-inch-square baking pan.
2. For filling, slice rhubarb in 1/2 - 1 inch pieces and toss with sugar, cornstarch and ginger. Set aside.

For crumbs

1. In a large bowl, whisk sugars, spices and salt into melted butter until smooth.
2. Then, add flour with a spatula or wooden spoon. It will look and feel like a solid dough. Leave it pressed together in the bottom of the bowl and set aside.

For the cake

1. In a small bowl, stir together the sour cream, egg, egg yolk and vanilla.
2. Using a mixer fitted with paddle attachment, mix together flour, sugar, baking soda, baking powder and salt.
3. Add butter and a spoonful of sour cream mixture and mix on medium speed until flour is moistened.
4. Increase speed and beat for 30 seconds.
5. Add remaining sour cream mixture in two batches, beating for 20 seconds after each addition, and scraping down the sides of bowl with a spatula.
6. Scoop out about 1/2 cup batter and set aside.
7. Pour the remaining batter into prepared pan.
8. Spoon rhubarb over batter.
9. Dollop the set-aside batter over rhubarb.
10. Using your fingers, crumble the crumble mixture all over the top.
11. Sprinkle it all over cake.
12. Bake cake until a toothpick inserted into center comes out clean of batter 45 to 55 minutes.
13. Cool completely before serving.