In a microwave-safe medium bowl, stir together the chocolate morsels and butter. Microwave on 50 percent power for 1 minute. Stir and, if necessary, microwave 30 seconds longer until mostly melted. Stir in corn syrup, vanilla, and 3 tablespoons hot water until mixture is completely smooth and well blended. Let glaze cool for 5 minutes. If it is too thick to flow readily, stir in a little more water until it is fluid enough to flow when spooned over the cake top. Spoon it over the cake, allowing the glaze to flow partially down the sides. Cake keeps, covered, 3 to 4 days.