Combine butter, milk, corn syrup, and vanilla in medium saucepan and heat over medium heat until butter is melted. Decrease the heat to low, add the chocolate, and whisk until melted. Turn off heat, add the powdered sugar, and whisk until smooth. Place the mixture over a bowl of warm water and dip the doughnuts immediately. Allow glaze to set for 30 minutes before serving.
Recipe courtesy of Alton Brown, 2004