Melt butterscotch morsels and chocolate over hot, not boiling, water. (Remove from heat just before boiling point is reached.)
Add Basic Sugar Syrup gradually to butterscotch-chocolate mixture, stirring until blended after each addition. (Mixture becomes glossy and smooth.)
Remove from hot water and cool until of spreading consistency. In warm weather, it may be necessary to set pan in ice water to cool quickly. Enough frosting for two 8" cake layers or a 13 X 9 X 2" cake.