1 stick butter
1 1/4 c. brown sugar
1 pkg. (single layer) yellow cake mix
1 1/2 - 2 c. rhubarb, finely chopped

Prepare cake mix according to package directions. Melt butter in cake pan (melt in oven while it is preheating). Remove pan from oven. Sprinkle brown sugar evenly over melted butter. Then sprinkle rhubarb over sugar. Pour cake mix over rhubarb and bake at 350 degrees for 35 to 40 minutes. When cake is done, remove from oven and let set a few minutes. Invert pan onto a plate larger than cake pan and let set a few minutes before lifting off the cake pan.