Rhubarb Upside-Down Cake

4 T butter
1 C brown sugar
2 C rhubarb
2 egg yolks (beaten)
3 T water
1 /2 tsp vanilla
1 C flour
1 tsp baking powder
1/2 tsp salt
2 egg whites (beaten)

Melt butter in a deep pan or cake pan.  Add rhubarb which has been cut into small pieces.  Sprinkle brown sugar over rhubarb.  Beat egg yolks, water, and sugar.  Add sifted flour, baking powder, and salt.  Add vanilla and stiffly beaten egg whites.  Pour over rhubarb and bake in moderate oven for 45 minutes.  Serve with milk or cream.