For caramel frosting, in double boiler, put 2 cups of the sugar, heavy cream, and salt. Put over gently boiling water. Upper pan should not touch water. Bring mixture to boiling. Don't stir.
Meanwhile, in heavy 8 inch skillet, cook 1/2 cup of the remaining sugar over medium high heat til sugar begins to melt, shaking skillet occasionally to heat sugar evenly. Don't stir. Reduce heat to low and cook about 5 minutes more or til sugar is melted and golden brown,stirring frequently.
Pour melted sugar over boiling cream mixture in double boiler. Clip a candy thermometer to side of double boiler. Cook to 236 (soft ball stage). Remove upper pan from water. Stir in butter or margarine. Let stand for 5 to 10 minutes or til slightly cool. Replace upper pan over gently boiling water and cook again to 236 (soft ball stage) Remove upper pan and let stand to cool while baking cake.
Preheat oven to 350. Grease and flour a 10 inch fluted tube pan. For cake, in large bowl, mix cake mix, sour cream, oil and remaining 1/2 cup sugar. Beat on low speed til moistened. Add eggs, one at a time, beating on medium speed til mixed. Beat for 2 minutes more. Pour batter into prepared tube pan. Bake about 45 minutes or til toothpick inserted near center of cake comes out clean.
Cool on wire rack for 10 minutes. Remove from pan and cool thoroughly.
While cake is still warm, frost cake with caramel frosting. If frosting slides off the warm cake, spread it back onto the cake. The frosting will eventually begin to harden. Makes 12 servings.