1. Bake the pie crust if needed and let it cool completely.

2. In a medium saucepan, whisk together the granulated sugar, cocoa powder, cornstarch, and salt.

3. Slowly pour in the milk while whisking until smooth, then whisk in the egg yolks.

4. Cook the mixture over medium heat, whisking constantly, until it thickens and begins to bubble.

5. Remove the pan from the heat and stir in the butter, vanilla extract, and chopped semi-sweet chocolate until smooth.

6. Pour the chocolate filling into the cooled pie crust and smooth the top.

7. Press plastic wrap directly onto the surface and chill for at least 4 hours, or until fully set.

8. Beat the heavy whipping cream with powdered sugar until soft peaks form.

9. Spoon or pipe the whipped cream over the chilled pie.

10. Finish with chocolate shavings before slicing and serving.