Whisk sugar, cocoa powder, and cornstarch in a large saucepan to combine.
Add eggs and egg yolks and whisk vigorously, making sure to get into corners of pan, until smooth.
Whisk in milk, vanilla, and salt.
Step 2 :
Place saucepan over medium heat and bring to a simmer, whisking often and making sure to get into corners of pan, 8–10 minutes (mixture should be bubbling and thickened).
Reduce heat and continue to simmer, whisking constantly, 3 minutes.
Let pudding cool 5 minutes, whisking every minute.
Step 3 :
Add chocolate to pudding and whisk until melted and fully incorporated.
Scrape filling into pie crust; smooth surface.
Chill until cold and set, at least 2 hours and up to 3 days.
Assembly :
Step 4 :
Stir gelatin and 1 Tbsp. water in a small bowl.
Let sit until solidified, about 5 minutes.
Microwave in 5-second intervals until melted, or set in a bowl of very hot water and let sit until melted.
Step 5 :
Using an electric mixer, beat cream, sugar, vanilla, and salt in a large bowl until soft peaks form.
Add melted gelatin and beat until medium peaks form.
Spoon whipped cream over filling, swirling as desired.