Heat sugar in a small skillet over high heat, stirring constantly with a wooden spoon until melted.
Lower the heat and stir until dark brown.
Carefully stir in 1/2 C of (heated) liquid called for in cake mix directions.
Stir until sugar dissolves.
Measure and add water, if necessary, to make 1/2 cup of liquid.
Prepare cake mix as directed substituting burnt sugar syrup for 1/2 C liquid.
Frost with vanilla or caramel frosting.