Add the milk to a medium saucepan, place over medium heat and bring to a boil.
In a bowl, whisk the egg yolks and sugar until light and fluffy. Add the cornstarch and whisk until smooth.
Whisk the hot milk gradually into the egg mixture until incorporated.
Pour the mixture back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened. You know it is done when you can run a wooden spoon along the bottom of the saucepan, and it leaves a clean line.
Remove from the heat and stir in the chocolate chips and butter until melted. Let cool slightly.
Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming.
Chill about an hour.
Once chilled, whip up the heavy cream until thick and you can overturn the bowl without any cream falling out. Remember not to over-whip.
Fold the whipped cream into the chocolate pastry cream.
Brownie and Whipped Cream Layers
Bake brownies according to the instruction on the box.
Whip the cream.
In each glass, place alternating layers of brownie bits, chocolate diplomat cream, and whipped cream. Garnish with shaved chocolate on top.