Cook rhubarb, water and sugar to a sauce, about 10 minutes, then add cornstarch and water. Simmer until thick, about 5 minutes. Add red food coloring (if you like). Cool. Whisk egg yolks until well blended. Whisk in sugar, cornstarch, salt and milk. Cook over low heat until thick, about 5-10 minutes. Add butter and vanilla. Top with wax paper to prevent skin formation and let cool completely. Make a 10" sponge cake using your favorite recipe. Bake in a pan. Cut into 1/2" cubes. Place 1/2 of the sponge cake cubes into a trifle bowl. Add 1/3 of the custard. Top that with 1/3 of the rhubarb. Do this again: cake, custard, rhubarb. Finally end with custard, rhubarb. Top with whipped cream.