In a medium, heavy saucepan, whisk together the egg yolks, all but 1/4 teaspoon of the sugar and the cornstarch over medium heat. Add the milk slowly, whisking constantly. Cook, whisking, until slightly thickened, about 2 minutes. Add the chocolate and butter, lower the heat and continue whisking until thickened, about 5 minutes. Transfer to a medium bowl. Press plastic wrap directly onto the surface of the custard and place the bowl in a larger bowl of ice water until room temperature, about 35 minutes. Divide the mixture among the pie crusts and refrigerate until set, about 1 hour.
Using an electric mixer, whip the heavy cream until stiff peaks form. In a small bowl, mix the cinnamon with the remaining sugar. Spread the whipped cream on the pies and sprinkle cinnamon-sugar on top.
The Do's and Don'ts of Cooking with Chocolate
