When the cake has been frosted and chilled (see baker's notes), it's time to glaze it. In a double boiler (or, a homemade one with a heatproof bowl set on top of a saucepan with an inch or two of water — the bowl shouldn't touch the water!) over medium heat, combine 4 cups confectioner's sugar, 1/4 cup light corn syrup, 6 tablespoons water, and 1 teaspoon pure vanilla extract. Whisk to combine and continue stirring occasionally until the mixture is smooth and homogenous. The icing should be fluid but not so warm that it will melt the frosting; the ideal final temperature of the mixture should be below 100 (F). Remove from heat and transfer the bowl to a wire rack to cool slightly.