Cook, cool and add the mixture to the crust. Place the bowl over a pot it fits snugly into, with a couple of inches of water in it. (Be sure the bottom of the bowl doesn’t touch the water.) Turn the heat to medium and whisking often, cook until the mixture is thick — it should coat the back of a spoon when it’s ready, about 15 minutes. Let it cool to room temperature and then pour it evenly over the crust. Refrigerate until it’s set, about 1 hour.