Cake

1. Preheat the oven to 350F/180C.
2. In the bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with a handheld electric mixer combine the butter and sugar for a couple of minutes until they are completely creamed together and lightened in texture.
3. Scrape the sides of the bowl and add one egg and mix until no trace of egg yolk remains. Add the second egg and mix again until no trace of egg yolk remains.
4. In a separate large bowl combine the cake flour, baking soda, baking powder, and salt. In another bowl or a liquid measuring cup with a pour spout combine the sour cream, whole milk, and vanilla extract.
5. Add 1/3 of the dry cake flour mixture to the butter mixture and mix until just combined then add half of the sour cream mixture and mix until just combined. Add another third of the dry ingredients followed by the wet and lastly the remaining dry, do not overmix the cake batter.

Topping

1. Heavily grease a 9-inch cake pan with nonstick baking spray, make sure it is a thick layer. Add the melted butter and swirl to coat the bottom of the pan and then sprinkle the granulated sugar on top. Your rhubarb stalks should be cut into small pieces, if your stalk is especially thick then I cut it in half then dice it into smaller pieces.  Add the rhubarb in an even layer and spoon the cake batter on top. I use an offset spatula to spread the cake batter into an even layer.
2. Bake the cake for 35 to 40 minutes, if you gently press the top it should spring back right away or if you insert a toothpick there should be no wet batter.  
3. Let the cake cool on a wire rack for five minutes then take an offset spatula and gently run it around the sides of the cake to loosen it from the pan. Now place a serving plate upside down on top of the cake and then invert the cake onto the plate to cool completely. The cake is best eaten the day it is made but leftovers will keep in an air-tight container for up to 3 days.