Step 1: Prepare the Egg Mixture

1. In a medium saucepan (off heat), whisk together egg yolks, granulated sugar, brown sugar, salt, and cornstarch.

2. Add 1 cup milk and beat with a hand mixer for 1 minute until smooth.

Step 2: Heat the Milk

1. In a separate microwave-safe bowl, heat the remaining 1 cup milk until bubbling at the edges (about 2-3 minutes).

2. If using a vanilla bean, slice it open, scrape out the seeds, and stir them into the warm milk.

Step 3: Combine and Cook

1. While whisking continuously, slowly pour the hot milk into the egg mixture.

2. Place the saucepan over medium heat and whisk constantly until the mixture thickens and bubbles (about 2-3 minutes).

3. Continue whisking for 15-45 seconds until the custard is thick. The temperature should reach 200°F.

Step 4: Finish and Chill

1. Remove from heat and stir in butter and vanilla extract (if not using a vanilla bean).

2. Strain the custard through a fine sieve into a bowl.

3. Cover with plastic wrap pressed directly onto the surface to prevent a skin from forming.

4. Refrigerate for at least 2 hours before using.