Combine melted butter, sugar, and a few drops red food coloring. Gently stir in the chopped rhubarb. Spread in a buttered 8-inch square cake pan.
Prepare the cake mixture and pour over the rhubarb.
Bake for 30 to 35 minutes or until cake is done.
Remove the cake from the oven and immediately run a spatula around edge of pan, invert onto a serving plate. Before lifting the pan from the cake, let the syrup drain onto the cake for a few minutes.
Cut while still warm; top each serving with whipped cream, if desired.