Mix together sugar and cornstarch. Add 2 egg yolks and half the milk. Beat well. Add remaining milk. Gradually add 1 c. water. If you want a yellow filling add a drop or two of yellow food coloring.
Cook on full power for 3 minutes in the microwave. Remove and stir well. Return to the microwave for 3 more minutes.
Remove and add 1 teaspoon vanilla and 1 tablespoon butter and stir to blend. If it is thicker than you want just add a tiny bit of milk till you get the consistency you want. Return to the microwave for one additional minute.