In a medium sauce pan, combine the sugar and corn starch. Using a whisk, slowly add the milk, blending it together until the corn starch is dissolved.
Add the egg yolks and whisk together until thoroughly combined.
Place pan over a medium flame, and whisking continuously, cook until it just begins to bubble.
Lower flame to low and cook for one more minute, whisking continuously.
Remove from heat and stir in butter and vanilla.*
Pour the pudding into a bowl. Place plastic wrap right on top of the surface of the pudding, smoothing out any air bubbles. This will prevent the pudding from forming a skin.
Chill for at least 3 hours.
Whisk again if you notice any lumps before serving.
Notes
*If for some reason, you pudding has become lumpy while cooking, you can pour it through a sieve and into the bowl. If I whisk it the whole time I am cooking it, my pudding never turns out lumpy.
Recipe by Premeditated Leftovers™ at https://premeditatedleftovers.com/?p=11525