Preheat the oven to 350 degrees. Grease a 9x9 inch baking dish and set aside.

For the cake layer, in a small bowl beat the sugar and butter together until light and fluffy, about 2 minutes. Add the egg and vanilla and stir well to combine. Add the flour, baking powder, and salt and stir until just incorporated, do not over mix. Add this batter to the bottom of your greased pan.

For the rhubarb layer add the rhubarb, brown sugar, and flour into a small bowl and toss well to combine. Evenly distribute the rhubarb layer over the cake layer. Bake the cake and rhubarb layers for 15 minutes.

While the cake is baking make your crumb topping. In a microwave safe bowl melt the butter. Add the flour, brown sugar, and salt. Stir to combine until the mixture resembles a thick cookie dough.

After the cake has cooked for 15 minutes, remove it from the oven and gently break off pieces of the crumb topping into large pieces (about the size of the end of your pinkie finger) and evenly distribute the crumb topping over the rhubarb layer.

Bake the cake an additional 30 to 35 minutes until a toothpick inserted into the center comes out clean. (Total baking time will be 45 to 50 minutes). Allow the cake to cook for at least an hour before serving warm or at room temperature.