In a small microwave safe bowl.microwave the chocolate on 50% power for 1 minute. Stir well. If necessary, continue microwaving on 50% power, stirring at 30 second intervals. Stop microwaving before the chocolate completely melts and let the residual heat finish the job. (Alternatively, melt the chocolate in a double boiler over simmering water, stirring frequently. Immediately remove from the heat. Sift the powdered sugar, and cocoa powder together into a medium bowl. Combine the corn syrup and 1/3 cup water in a small saucepan. Heat to just boiling, stir well. Stir the syrup mixture into the powdered sugar mixture just until well blended and completely smooth. Stir in the melted chocolate and vanilla until completely smooth. Thin the glaze to a spreadable consistency with hot water, a few drops at a time until completely smooth. Set the glaze in a large bowl half full of hot tap water to help keep glaze warm while you work. Reheat it to warm in a saucepan or in a microwave over if it cools completely) If the glaze thickens upon standing, stir in a few more drops of hot water until thoroughly blended; be sure to blend in thoroughly, so the glaze maintains its luster. Using a table knife, spreader or small palette knife, cover the top of each cookie witha smaooth even layer of glaze. Let the cookies stand on wire racks until the glaze completely sets, at least 1 1/2 hours and preferably several hours. Store in a single layer or layered with wax paper in an airtight container.