In a bowl, combine rhubarb, cornstarch and Muscovado sugar. In another bowl, sift the cake flour, baking powder and salt and set aside.
In a small skillet, mix the brown sugar and 50 g of butter. Place over medium heat and stir until smooth and bubbling. Keep warm.
With a hand held mixer beat 230 g butter and caster sugar until light. Add lemon zest and vanilla extract. Cream until light and fluffy, scraping down sides often.
Add eggs, one at a time. Mix to combine after each addition.
Beat in the sour cream and lemon juice (it may look curdled at this point). Add the flour mixture, 1/3 cup at a time, until combined.
Line bottom and sides of 9-inch spring-form pan with parchment paper. Wrap two layers of foil in the pan and place it on a baking sheet (to prevent leaking).
Pour the brown sugar mixture into the prepared spring-form pan, then add the rhubarb. Pour the batter over everything, smoothing it over.
Bake the cake in the oven at 160?C for 1 hour or until a cake tester comes out clean.
Cool (with the foil turned down a bit) on a wire rack for only 15 minutes. Run a knife around the cake, place a plate on top and turn it upside down. Removed the foil and released the spring-form sides carefully.