1. Pre-heat oven to 325°F. Grease an 8-inch square baking pan. Set aside.
2. Make the cake: in the bowl whisk the sour cream, egg, egg yolk, and vanilla.
3. In the bowl of a stand mixer fitted with paddle attachment mix the flour, sugar, baking soda, baking powder, and salt. Add in the butter and 1/4 of the sour cream mixture until combined. Then add in the remaining sour cream mixture in two batches, beating for 30 seconds after each addition. Set aside.
4. Make the rhubarb filling: in a bowl combine the rhubarb, sugar, cornstarch and 1/2 teaspoon Ginger. Set aside.
5. Assemble the cake: pour half the cake batter into the prepared pan. Top with the rhubarb filling, then spoon the remaining batter on top the rhubarb.
6. Make the crumb topping. In a bowl combine the sugars, salt, melted butter, 1 teaspoon Cinnamon and 1/2 teaspoon Ginger. Then stir in the flour until combined. Using your fingers press the crumbs into 1/2 inch chunks and cover the cake.
7. Bake for 45-55 minutes or until a toothpick comes out clean. Cool completely on a wire rack before cutting.
LIKE: Better Batter and King Arthur Gluten Free Multi-Purpose Flour. HINT: If using King Arthur Gluten Free Multi-Purpose Flour or a custom blend, you will need to add Xanthan Gum. Cookies/Bar Cookies, Breads, and Pizza use 1 teaspoon of Xanthan Gum for every 1 Cup GF Flour. If using Better Batter, you will need increase the liquid in the recipe by 50%. Add in small amounts until the desired moisture is reached.
OTHER: Brown Rice, Oat, Tapioca, Buckwheat, Almond, Teff, and Sorghum Flour.