Preheat oven to 325˚F. Grease an 8-inch square baking pan. Combine the rhubarb, 4 tablespoons of the sugar, 1/2 teaspoon of the ginger, and the cornstarch in a mixing bowl. Set aside.

To make the crumble topping, melt 8 tablespoons (1 stick) of the butter in a small saucepan. In a large bowl, stir together 5 tablespoons of the sugar, the brown sugar, cinnamon, remaining 1/2 teaspoon of ginger, and a pinch of salt. Whisk in the melted butter until smooth. Stir in 1 3/4 cup of the flour, and you'll have a solid dough. Set aside.

In a small bowl, stir together the sour cream, egg, egg yolk, and vanilla extract. Using a mixer with a paddle attachment, mix together the remaining cup of flour, last 8 tablespoons of sugar, baking powder, and remaining 1/4 teaspoon of salt. Add last 6 tablespoons of butter, then 1 tablespoon of the sour cream mixture. Mix until the flour is moistened. Increase the speed and beat for 30 seconds. Add the rest of the sour cream mixture in two batches, beating 20 seconds after each addition.Scrape down the sides of the bowl with a spatula. Scoop out 1/2 cup of the batter and set aside.

Scrape the batter into the prepared pan. Spoon the rhubarb over the batter, then dollop the reserved batter on top of the rhubarb.

Use your fingers to break the crumb topping dough into big crumbs about 1/2-inch to 3/4-inch big. Sprinkle them over the rhubarb and batter. Bake for 45-55 minues, until a toothpick inserted into the center comes out clean of batter - it might still be moist from the rhubarb. Cool completely before serving.