Spray (or butter) a 9-inch round cake pan (that is at least 2-inches deep) with nonstick spray then dot with a half stick of butter that has been cut into pieces.
Toss the rhubarb again, and place rhubarb on its side in a mosaic like pattern, using the butter to hold it up. Use the small pieces of rhubarb to fill in the gaps. Spoon a little (not all) of the rhubarb sugar over top. However this step is optional.
Preheat oven to 350 °.
Whisk together the flour, baking powder, and kosher salt.
In the bowl of your stand mixer, beat remaining stick of butter and sugar in your mixer on medium speed until light in color and fluffy.
Add in the orange zest, orange juice and the vanilla extract .
Crack in the eggs, one at a time, until incorporated, scraping down sides of bowl in between.
Alternate adding in the 1/3 of the flour mixture with 1/3 of the sour cream and repeat, mixing until smooth.
Spread the cake batter over the rhubarb, then sprinkle with the crumble topping evenly over batter.
Place the cake on a foil-lined sheet pan (to catch any spill over) and bake for 45 to 55 minutes or until a toothpick inserted into the center comes out clean and top springs back when touched.
Allow the cake to cool 10 minutes before running a knife around edge of the cake, and inverting it onto your favorite platter or cake stand.
Let cool completely before serving with whipped cream or vanilla ice cream.