In a medium bowl, combine the sugar, salt, and cornstarch.
In a medium saucepan, whisk together the milk, half-and-half, and egg yolks.
Place the saucepan over medium to medium-high heat and whisk in the sugar mixture. Cook until the mixture bubbles and thickens, stirring often. Once the milk mixture feels pudding-like (after about 8 to 10 minutes), remove the pan from the heat.
Add the butter and vanilla and whisk until the butter is melted. Add the chocolate chips and whisk until the chocolate and milk mixture are completely combined (no white showing).
Pour the chocolate filling into the prepared pie crust.
Place a piece of plastic wrap over the top of the pie, making sure it touches the surface of the filling.
Refrigerate until the filling has cooled completely and is set.
Add the heavy cream, powdered sugar, and vanilla to a large mixing bowl. Beat until soft peaks form.
Spread the whipped cream over the top of the cooled pie. I like to put all the whipped cream in the center and spread it out towards the edges, and leave a 1/2-inch gap around the edge so you can see the filling.
Sprinkle the whipped cream with mini chocolate chips. Refrigerate until ready to serve.
If half-and-half is not available, you can substitute the same amount of milk.