1. Preheat oven to 325ºF. Butter and flour an 8″ square baking pan.
2. In a large bowl, toss the rhubarb with sugar, cornstarch and ginger. Set aside.
3. Make the crumbs by placing sugar, spices, salt, butter and flour in a bowl. Using clean hands, pinch the ingredients together until they form a solid dough but is still a bit crumbly. Set aside.
4. To make the cake, in a mixer fitted with a paddle attachment mix the sour cream, egg, egg yolk, vanilla and butter. Then, in a separate bowl whisk together the flour, sugar, baking soda, baking powder and salt. With the mixer on low, add the flour mixture bit by bit until it is totally incorporated, but not overbeaten. The mixture should be totally smooth and slightly sticky.
5. Pour ¾ of the batter into the greased 8″ baking pan and pat it down evenly. Distribute the rhubarb over the top in one single layer. Dollop the remaining cake batter over the rhubarb in a few spoonfuls and make sure it is somewhat even.
6. Using clean hands, crumble the large crumb dough over the top of the dough. The chunks should be anywhere from ½″ to ¾″ in size. The whole cake should be completely covered. Bake for 45-50 minutes or until a cake tester inserted in the center comes out clean. Cool and serve.