Preheat oven to 325°F. Grease and flour a 8 x 8 square cake pan.
For the cake, mix together the sour cream, whole egg, egg yolk, and vanilla in a bowl. In the bowl of an electric mixer fitted with a paddle attachment, combine the flour, sugar, baking soda, baking powder, and salt. Add the butter and one spoonful of the sour cream mixture. Mix until flour is moistened, about 30 seconds. Add the rest of the sour cream mixture in two additions, mixing well after each addition. Pour cake batter into prepared cake pan.
For the rhubarb filling, cut rhubarb into ½ inch pieces. Toss with cornstarch and spices. Spoon over the cake batter.
For the crumb topping, stir together the sugars, spices, salt and butter until smooth. Stir in flour with a spatula. It will look like a solid dough. Break it apart with your fingers into large "crumbs". Sprinkle over cake. Bake for 45 to 55 minutes until toothpick inserted into the center comes out clean. Cool completely before serving.