Delicious served warm with ice cream.
Bakers frequently err by failing to toss chopped rhubarb in flour before folding it into batter, allowing heavy stalks to sink directly to the bottom of the pan during baking. This cake relies on a 9x13 inch metal or glass baking dish to distribute the heat evenly across a wide surface area, preventing the center from staying raw while the edges overbake. Buttermilk provides necessary acidity that reacts directly with the teaspoon of baking soda to generate carbon dioxide gas, lifting the heavy crumb structure without requiring excessive mechanical leavening. Creaming the butter and brown sugar together incorporates tiny air pockets that expand in the oven at 350 degrees F, establishing the initial framework for the crumb. Tossing the two cups of diced rhubarb with one tablespoon of all-purpose flour creates a friction-based coating that prevents the pieces from precipitating through the batter. The resulting texture depends heavily on maintaining these trapped air bubbles; overmixing the flour after the buttermilk is added will develop excess gluten networks, yielding a tough and rubbery cake instead of a tender one. Mixing the crumb topping with cold butter cut into brown sugar and ground cinnamon produces a coarse streusel that bakes into a crisp crust within the forty-five minute baking window. Cool the finished dessert completely on a wire rack before slicing to allow the starches to fully set and firm up the interior structure.