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

For crumbs

• In a large bowl, whisk sugars, spices and salt into melted butter until smooth.
• 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

• In a small bowl, stir together the sour cream, egg, egg yolk and vanilla.
• Using a mixer fitted with paddle attachment, mix together flour, sugar, baking soda, baking powder and salt.
• Add butter and a spoonful of sour cream mixture and mix on medium speed until flour is moistened.
• Increase speed and beat for 30 seconds.
• 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.
• Scoop out about 1/2 cup batter and set aside.
• Pour the remaining batter into prepared pan.
• Spoon rhubarb over batter.
• Dollop the set-aside batter over rhubarb.
• Using your fingers, crumble the crumble mixture all over the top.
• Sprinkle it all over cake.
• Bake cake until a toothpick inserted into center comes out clean of batter 45 to 55 minutes.
• Cool completely before serving.