Chocolate Pastry Cream
Combine the milk, sugar, chocolate, and vanilla in a large nonstick saucepan over medium heat. Whisk to incorporate the chocolate. When the mixture comes to a gentle boil or simmers after about 5 minutes, take 1 cup of the chocolate mixture and add it to the yolks. Whisk well. Slowly whisk the yolks into the chocolate mixture. Cook over medium heat until it thickens slightly, about 4 to 5 minutes. Dissolve the cornstarch in the water. Slowly stir into the hot liquid. Continue to cook for 2 minutes. Remove from the heat and stir in the butter. Pour into a glass bowl. Cover with plastic wrap, pressing the wrap down on the surface of the pastry cream to prevent a skin from forming. Cool completely and chill for at least 4 hours.