Prep: Preheat oven to 350℉. Butter a 9-inch cake pan WELL. I also like to use a parchment paper round, and butter over it, and give a light spray with nonstick spray, to ensure the fruit doesn't stick in the pan. Cut the 4 tbsp of butter into small chunks and scatter them in the bottom of the pan. Set aside. Stir together the chopped rhubarb and the sugar and let rest while you make the batter.

Make the Cake Batter: Cream softened butter in a large mixing bowl for about 30 seconds to 1 minute. Add the sugar and mix on high until creamed together, about 1 minute. Mix in the eggs on medium-high speed, beating until well combined and the batter is smooth. Add the orange zest & juice, and vanilla and mix on medium speed until just combined. Mix in the sour cream until just combined & smooth. Mix in the dry ingredients - flour, baking powder, & salt - on low speed. When a few streaks of flour remain, pour in the milk and finish mixing on low speed. Spread the rhubarb in an even layer in the buttered cake pan, nestling the pieces close together. Pour batter over the rhubarb, spreading it evenly.

Make the Crumb Topping: In a small bowl, melt the butter. Stir in remaining crumb ingredients until a crumbly mixture forms. Sprinkle the crumbs over the cake batter.

Bake: Bake for 45-55 minutes. When done, a toothpick inserted into the center of the cake will come out with moist crumbs, but not wet batter. Let the cake cool in the pan for 10 minutes, then run a knife around the edge to loosen and then invert the cake onto a cooling rack that has a cookie sheet underneath (some rhubarb juice will spill out). Don't let it sit longer than this because the rhubarb can begin to stick in the bottom of the pan. If the cake seems to be sticking, gently but firmly tap on the bottom of the cake pan with the butt of a butter knife while the cake pan is flipped upside down on the cooling rack. Let cake cool before cutting and serving. You can speed this up by moving it, uncovered, to the fridge for about 20 minutes.

Store: Store at room temperature for up to 3 days, tightly covered with foil, plastic wrap, or in a large tupperware container. Store in the fridge, well covered, for up to 6 days.