**Preparation:** Set a bowl with a fine-mesh sieve on top to strain and cool the pastry cream once it’s cooked.
**Mix Cornstarch and Sugar:** In another bowl, whisk together cornstarch and sugar. Add eggs and whisk until smooth.
**Heat Milk Mixture:** In a saucepan, combine milk, salt, and vanilla extract. Heat over medium-high until just below boiling, stirring occasionally.
**Combine and Cook:** Slowly add a third of the hot milk to the egg mixture, whisking constantly. Then pour this mixture back into the remaining milk. Cook over medium heat, whisking until thick like lightly whipped cream. Remove from heat just before it boils and pour through the sieve into a bowl.
**Add Chocolate and Butter:** Stir in chocolate until melted. Add butter one tablespoon at a time, stirring until smooth. Let it cool for 10 minutes, then press plastic wrap onto the surface and refrigerate for at least 1 hour.