Preheat your oven to 325°F. Grease and line one 8-inch-square baking pan. Set it aside.

For the Filling:

1. In a small bowl, mix together the rhubarb, sugar, cornstarch, and ginger. Set aside.

For the Crumb Topping:

1. Whisk the sugars, spices, and salt in a large bowl until completely mixed together and uniform. Add the flour and whisk to combine.
2. Add the melted butter and mix with your hand until a crumbly, semi-solid mixture forms. Push it all into the bottom of the bowl, and leave it pressed. Set aside.

For the Cake:

1. Whisk together the sour cream, egg, egg yolk, and vanilla in a small bowl until completely smooth.
2. In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, baking soda, baking powder, and salt.
3. Add the butter and a spoonful of the sour cream mixture. Mix on medium speed until the flour is a little moistened. Increase speed and beat for 30 seconds.
4. Add the rest of the sour cream mixture in two batches, beating for 20 seconds after each addition, scraping down the sides of bowl with a spatula. Mix until a thick and creamy batter develops.

To Assemble and Bake:

1. Scoop out about half of the batter into the prepared pan, and spread it evenly on the bottom. Scatter the rhubarb mixture over the batter. Spread the rest of the batter over the rhubarb.
2. Using your fingers, break the dough-like mixture into larger crumbs, and sprinkle evenly over the cake. It will be a thick layer of crumb topping.
3. Bake until the top starts to brown, and a toothpick inserted into center comes out clean with a few crumbs, about 45-50 minutes.
4. Cool completely in the pan before serving.