Butter Prep: Slice butter and let it sit at room temperature for at least 1 hour to warm up.
Pudding Setup: Measure pudding ingredients, set aside 1/3 of milk. In a pot, mix remaining milk, sugar, vanilla extract, egg yolks, and heat.
Starch Slurry: Mix reserved milk with cornstarch in a separate bowl.
Cook Pudding: Once milk mixture is almost boiling, add the starch slurry while stirring. Stir on low heat until thickened, then remove from heat.
Cooling: Cover pudding with plastic wrap, cool to room temperature for 60-90 minutes.
Pastry Cream: Whip room-temperature butter, then gradually add cooled pudding (one tablespoon at a time) and mix to a creamy consistency.
Adjust Taste: Add powdered sugar if needed, mix briefly to combine.
Chill Cream: Refrigerate for at least an hour before use.