Ingredients:For the rhubarb filling:1/2 pound rhubarb, trimmed1/4 cup (50 grams) sugar2 teaspoons (15 grams) cornstarch1/2 teaspoon ground gingerFor the crumbs:1/3 cup (65 grams) dark brown sugar1/3 cup (65 grams) granulated sugar1 teaspoon ground cinnamon1/2 teaspoon ground ginger1/8 teaspoon salt1/2 cup (4 ounces or 113 grams) butter, melted1 3/4 cups (225 grams) cake flour (I was out and used all-purpose and it worked great)For the cake:1/3 cup (80 grams) sour cream1 large egg1 large egg yolk2 teaspoons (10 ml) vanilla extract1 cup (130 grams) cake flour (ditto on the all-purpose flour–worked just fine)1/2 cup (100 grams) sugar1/2 teaspoon baking soda1/2 teaspoon baking powder1/4 teaspoon salt6 tablespoons (85 grams) softened butter, cut into 8 pieces.Cooking Instructions:Preheat oven to 325 degrees. Grease an 8-inch-square baking pan. For filling, slice rhubarb 1/2 inch thick and toss with sugar, cornstarch and ginger. Set aside.To make crumbs [this step now updated, see comment #150] in a large bowl, whisk sugars, spices and salt into melted butter until smooth. 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.To prepare 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.Scrape remaining batter into prepared pan. Spoon rhubarb over batter. Dollop set-aside batter over rhubarb; it does not have to be even.Using your fingers, break topping mixture into big crumbs, about 1/2 inch to 3/4 inch in size. They do not have to be uniform, but make sure most are around that size. Sprinkle over cake. Bake cake until a toothpick inserted into center comes out clean of batter (it might be moist from rhubarb), 45 to 55 minutes. Cool completely before serving.Source: