1. Whisk together the milk, heavy cream, and cornstarch in a medium saucepan until the cornstarch is well mixed into the liquid.
2. Add the egg yolks, sugar, cocoa powder, and salt.
3. Cook over medium heat until the mixture begins to boil, stirring constantly.
4. Remove the pan from heat and add the butter and vanilla. Stir until the butter is melted.
5. Allow the filling to cool for around five minutes, then pour it into the prepared pie crust.
6. Place the pie in the refrigerator to chill for at least 4 hours or up to overnight.
7. Spread the whipped topping over the top of the pie and garnish with mini chocolate chips.