Place cornstarch in a small measuring cup. Remove 1/4 c. Milk from 2 c. Pour over cornstarch. Do Not Stir. Let sit while you prepare the rest.
Put milk and chocolate in an 8 cup glass pitcher, and microwave on high for about 3 minutes, remove, beat with whisk or fork, and continue heating and stirring until you have melted all the chocolate. If a few flecks remain, it's okay. Beat egg yolks with sugar. Pour a bit of the hot chocolate/milk mixture into the beaten egg yolks, and stir. Continue adding hot chocolate to the egg yolks until you have added about 3/4 c. Of the mixture. Then add this back into the large container of hot chocolate mixture.
Gently stir the cornstarch mixture and it will appear very stiff. Continue to stir and the cornstarch will suddenly blend smoothly into the milk. Add this smooth mixture to the chocolate-egg mixture and stir well. Return to microwave, and heat on high for about l minute. Scrape sides well, beat mixture, and continue heating for l minute intervals or less. Mixture will seem to thicken quickly at the edges, and may even seem to curdle. Don't panic. Remove, and beat vigorously, and it will smooth. When it is at about the consistency of a light mayonnaise, fluffy and pudding-y, remove from heat, and stir in butter. Voila!