In a mixing bowl, whisk the egg yolks, sugar, cornstarch, and salt until pale, smooth, and slightly thickened.

Slowly drizzle a small amount of the warm milk mixture into the egg mixture while whisking constantly to gently raise the temperature.

Continue adding the warm milk gradually until the egg mixture feels warm and fully blended.

Pour the tempered mixture back into the saucepan and cook over medium heat while stirring continuously with a silicone spatula.

Stir slowly across the bottom and sides of the pan to prevent sticking as the custard thickens into a silky consistency.

Cook until the custard coats the back of a spoon and leaves a clean line when traced with a finger.

Remove the saucepan from the heat immediately once thickened to avoid curdling.

Stir in the butter and vanilla extract until fully melted and fragrant.

Strain the custard through a fine mesh strainer into serving bowls for an ultra-smooth texture.

Cover the surface with plastic wrap touching the custard directly to prevent a skin from forming.

Chill for at least 2 hours for a cold dessert or serve slightly warm for a softer texture.