Dissolve flour and cornstarch in 1/4 cup of milk to prevent lumps.
Pour into the top of a double boiler over simmering water OR a saucepan over a low flame. Whisk in the rest of the ingredients (one at a time) except for 1/2 tsp vanilla extract. Cook for 20 minutes, stirring constantly to prevent burning the bottom. Then whisk for an additional 1-2 minutes off the stove before adding the remaining vanilla. Chill for 3 hours.