Prepare Base
Combine sugar, corn starch, and cocoa powder.
Whisk in cream until smooth.
Whisk in milk.
Whisk in egg yolks one at a time.
Heat over medium-low, whisking constantly until thickened (up to 10 minutes).
Increase heat to medium if needed, stirring constantly.
Once bubbling, stir for 5 seconds, then remove from heat.
Finish Pastry Cream
Whisk in chocolate until melted.
Whisk in butter and vanilla until incorporated.
Cover surface with plastic wrap.
Refrigerate at least 4 hours.
