In a medium saucepan, bring about an inch of water to a boil, and then reduce the heat to medium. Place a medium sized bowl on top of the simmering water, and add the butter, milk, corn syrup, and vanilla to it. Once the butter as melted, add the chocolate chips in, whisking until the chocolate has melted. Remove the bowl from the heat, and whisk in the icing sugar until smooth. If the glaze has some lumps in it, return to the heat, and over heat, whisk the lumps away.
Recipe courtesy of Alton Brown of the Food Network.