In a large bowl, mix cornstarch, sea salt and sugar with a whisk and place on top of a pot of boiling water. Slowly stir in cream, milk and beer, mixing till combined; stir occasionally until mixture is thick and smooth, about 12 – 15 minutes total. Remove from the heat and stir in vanilla. Let custard come to room temperature, then cover and refrigerate until ready to use. Can be made 2 – 3 days in advance.