1. Make cake mix according to instructions on the back of the package in greased and floured 9x13 pan. Then let cool.
2. Put butter, white sugar, brown sugar, buttermilk, and baking soda in a saucepan with high sides. Stir every few minutes. When the mixture reaches 236 degrees or balls slightly when put into cool water it is done cooking.
3. Remove from heat and spread on COOL cake, the mixture will cool and become firm.