Prepare an ice-water bath; set aside.
In a medium bowl, whisk together egg yolks, sugar, and salt until well combined.
Pour milk into a heavy-bottomed, nonreactive saucepan. If using the vanilla bean, scrape seeds from pod halves into the milk using the tip of a knife. Add pod halves. Place over medium heat until small bubbles appear around the edges of the pan.
Remove from heat and slowly pour milk mixture into egg mixture, whisking constantly. Pour mixture back into the saucepan and cook over medium-low heat, stirring constantly with a spatula, until slightly thickened and coats the back of the spatula, about 5 minutes. Pour into a medium bowl and set bowl in ice water bath. Let cool to room temperature, stirring occasionally. Stir in vanilla extract, if using.
Pour custard through a fine-mesh sieve into another bowl, cover, and refrigerate until serving.