Whisk sugar, cornstarch, cocoa powder, and salt in a medium saucepan. Whisk egg yolks, milk, and cream in a large bowl; scrape in vanilla seeds and add pod. Gradually whisk milk mixture into dry ingredients, whisking until smooth.
Bring mixture to a boil over medium-high heat, whisking constantly. Reduce heat to medium and continue cooking, whisking, until mixture is thick enough to coat a spoon, about 4 minutes. Remove from heat and whisk in chocolate, butter, and vanilla extract (mixture will thicken). Strain through a fine-mesh sieve into another large bowl.
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