Cook rhubarb over low heat until juice begins to run, add sugar and marshmallow and mix well. Simmer about 10 minutes and pour into greased pan. Sift flour, baking powder and salt together. Cream shortening with sugar until fluffy. Add egg yolks and flavorings and beat thoroughly. Add sifted dry ingredients and milk alternately in small amounts beating well after each addition. Beat egg whites until stiff but not dry and fold into batter. Pour over rhubarb mixture and bake in a 350 degree oven for 40 to 50 minutes. Loosen cake from sides and bottom with a spatula and turn out onto a cake plate. If marshmallows are very fresh, do not add to rhubarb until just before the cake batter is poured on. Serves 8.