In a small skillet, melt sugar over medium-high heat until medium golden, about 3 minutes. Remove skillet from heat. Working quickly, drop walnut halves, one at a time, into the melted sugar. If the sugar hardens, return skillet to low heat, and stir several minutes. Using a fork, turn walnuts until they are evenly coated. Transfer walnuts to a wire rack to cool completely. Use to decorate cake.