Prepare the Egg Mixture:
In a medium saucepan (off heat), whisk together egg yolks, granulated sugar, brown sugar, salt, and cornstarch.
Add 1 cup milk and beat with a hand mixer for 1 minute until smooth.

Heat the Milk:
In a separate microwave-safe bowl, heat the remaining 1 cup milk until bubbling at the edges (about 2-3 minutes).
If using a vanilla bean, slice it open, scrape out the seeds, and stir them into the warm milk.

Combine and Cook:
While whisking continuously, slowly pour the hot milk into the egg mixture.
Place the saucepan over medium heat and whisk constantly until the mixture thickens and bubbles (about 2-3 minutes).
Continue whisking for 15-45 seconds until the custard is thick. The temperature should reach 200°F.

Finish and Chill:
Remove from heat and stir in butter and vanilla extract (if not using a vanilla bean).
Strain the custard through a fine sieve into a bowl.
Cover with plastic wrap pressed directly onto the surface to prevent a skin from forming.
Refrigerate for at least 2 hours before using.