Place cornstarch in a medium-sized bowl. Add ½ cup of the milk and whisk until cornstarch is dissolved.
½ Cup Cornstarch, ½ Cup Whole Milk (measure #1)
Add the egg and egg yolks to the cornstarch/milk mixture. Whisk lightly until combined.
1 Large Egg, 4 Large Egg Yolks
Combine 1½ cup milk (measure #2) and granulated sugar in a saucepan. Heat over medium heat until the mixture reaches a low boil. Remove from heat.
1½ Cup Whole Milk (measure #2), ½ Cup Granulated Sugar
Gradually pour ½ cup of the hot mixture into the egg mixture while whisking constantly. This steady whisking is crucial - it helps gently raise the temperature of the eggs without scrambling them.
Return the remaining milk/sugar mixture to a simmer over medium-low heat. Slowly pour the egg mixture into the saucepan, whisking constantly.
Continue whisking vigorously until the mixture becomes thick and forms ribbons when lifted with the whisk, about 3 to 5 minutes. Consistent whisking ensures the eggs are evenly heated and prevents them from curdling. If the eggs cook too quickly you’ll end up with a lumpy texture instead of a smooth pastry cream.
Remove saucepan from heat. Add the butter and vanilla. Whisk until all the butter is melted.
4 Tablespoons Unsalted Butter, 1 Tablespoon Pure Vanilla Extract
Transfer the pastry cream to a heatproof bowl, then press a sheet of plastic wrap directly onto the surface of the cream. This prevents a skin from forming as it cools by blocking air exposure.
Store in refrigerator until cooled.
Notes
Store pastry cream in refrigerator until ready to use.