Sift the sugar into a bowl, then add the vanilla extract. Gradually stir in the hot water until the mixture is the consistency of thick cream. Cover each brownie with icing.
Make a second batch of icing, this time adding a few drops each of red and yellow food coloring until the icing is orange. Put the icing into a piping bag (if you don't have a piping bag, you can put the icing into a plastic bag and cut a very small hole in the bottom corner). To make a cobweb design: make fine, concentric circles with the colored icing starting from the center and working outwards. Using a toothpick, pull the colored icing from the center to the outside edge, dividing the brownie into quarters then repeat to divide into eighths.