Step 1 Place the butter in a strainer set over a bowl. Place the bowl over a larger bowl of ice water to form an ice bath.
Step 2 In a heavy saucepan, whisk together the milk, maple syrup, salt and vanilla bean. Cook over medium-high heat, striring frequently, until the mixture comes to a boil, 5 to 7 minutes.
Step 3Meanwhile, whisk together the eggs, egg yolks and cornstarch.
Step 4Whisk one-half cup of the boiling half-and-half into the egg mixture to temper the eggs, then slowly stir the egg mixture into the hot liquid. Increase the heat to high and cook, whisking constantly (and scraping the sides and bottom of the pan), until the mixture thickens and comes to a boil.
Step 5Immediately remove the pan from heat and pour the mixture over the butter in the strainer. Strain the custard, then gently stir until the butter is completely incorporated. This makes about 3 cups custard.
Step 6Divide the custard between serving cups or ramekins. Place a sheet of plastic wrap over the surface of each custard to prevent a skin from forming, and set aside until cooled.