Pre-heat your oven to 350 (F). Grease a 9 inch round cake pan. In the bottom of the greased cake pan add the diced rhubarb and spread it out in an even layer. Over the top of the rhubarb sprinkle the brown sugar then set the pan aside.
In your stand mixer or using a hand mixer in a large bowl, cream the butter for several minutes until the texture is fluffy and the color of the butter has lightened a lot. Add in the cup of sugar and continue blending for several minutes until the butter and sugar is creamy.