Place a small saucepan over medium heat until the milk begins to steam and small bubbles form along the edges. Do not let it boil.
In a bowl, whisk together egg yolks, sugar, sea salt, and cornstarch until smooth and pale.
Slowly whisk a small ladle of hot milk into the egg mixture. Do not add all the milk!
Pour the egg mixture back into the saucepan. Cook over low heat, stirring constantly, until the custard thickens enough to coat the back of a spoon. To check, run your finger across the back of the spoon—if it leaves a clear path, it’s ready. Avoid boiling. Stir in the remaining vanilla bean paste (or vanilla extract).
Remove from heat and let cool slightly.. For extra smoothness, strain through a fine-mesh sieve into a bowl. Press plastic wrap directly onto the surface to prevent a skin. Refrigerate until chilled.
Drizzle over cakes, tarts, or fruit, or use as a dip for cookies. This rich, velvety sauce enhances any dessert!