Rhubarb Upside-Down Cake
["3 c. sliced rhubarb", "1 1/2 c. miniature marshmallows", "3/4 c. sugar", "1/2 c. soft butter or margarine", "1 c. sugar", "2 eggs", "1 3/4 c. sifted all-purpose flour", "3 tsp. baking powder", "1/2 tsp. salt", "1/4 c. milk", "1 tsp. vanilla"]
["Grease a 13 x 9 1/2 x 2-inch pan.", "Oven temperature is 350\u00b0."]
["rhubarb", "marshmallows", "sugar", "butter", "sugar", "eggs", "flour", "baking powder", "salt", "milk", "vanilla"]