Preheat oven to 325°F. Line the bottom of a 9 inch spring-form pan with parchment paper and then grease the sides of the pan and the parchment paper with butter. Next, wrap two layers of aluminum foil around the base of the pan. Place the spring-form pan on a rimmed baking sheet.
Using medium size mixing bowl, combined rhubarb and ½ cup sugar and stir well. Sprinkle cornstarch into mixture, while stirring gently until thoroughly combined.
Place ½ stick of butter into a small sauce pan over medium-low heat and slowly melt it. Add the brown sugar, whisk until smooth, about 2 minutes. Next, combine combine cake flour, baking powder, and salt in a small mixing bowl. Gently stir together until well combined.
Using the paddle attachment of a stand mixer, whip the 2 remaining sticks ( one cup) of butter for about two minutes. Lower the mixer speed to low and add remaining 1 cup of sugar. Turn mixer off and use your fingers to blend in the lemon zest until completely mixed in and the mixture is uniform in color. Next, add the vanilla and cream this mixture together in the sand mixer, for about four minutes. Reduce speed to low and add the eggs, one at a time. Use a rubber spatula to scrape sides and bottom of bowl after the addition of each egg. Add the sour cream and 1 tablespoon of the lemon juice. It's normal for the mixture to curdle at this point. With the mixer speed on low, add the flour mixture a little at a time and scrape the sides and bottom of the bowl as you add the dry mixture.
Use a tablespoon or rubber spatula and pour the brown sugar and butter mixture into the prepared spring-form pan. It may be necessary to gently reheat the mixture prior to doing this. Next, spoon the rhubarb pieces and their juices onto the brown sugar and butter base. Finally, spoon the cake patter over the rhubarb, making sure to evenly distribute the batter and smooth it out with the back of a spoon or a rubber spatula.
Bake for 70-75 minutes or until firm and a tooth pick comes out clean after being inserted in cake's center. Cool on a wire wrack for 15 minutes and then use a paring knife to loosen the edges of the cake from the sides of the pan. Place a serving platter on top of the spring-form pan and invert while the cake is still warm. Waiting too long to release the cake from the pan may cause it to stick to the sides of the pan.
Use cake flour if possible for making this rhubarb cake. It will give you a lighter crumb and taste. If you don't have cake flour, all-purpose flour will work as well.