Separation - The Most Common Toffee Pitfall: Toffee sometimes separates during cooking or when spread onto the pan, leaving a buttery layer on the surface and a thicker mixture underneath. This is caused when the liquid in the mixture evaporating too quickly, or stirring the mixture too fast leading to the liquid and fat separating.
Salt in the recipe seems to stabilize the mixture. Use salted butter, or if you use unsalted butter add ¼ teaspoon of salt per stick of butter.
Be patient because candy takes a long time to cook. Don't rush the process by turning up the heat. Stir slowly and gently during the final stages of cooking.
If the butter toffee does separate:
A. Continue to stir the mixture. The toffee may remix on its own. Lower the heat, slowly stir, so not to splash yourself with "really hot butter" until it comes back together.
B. If gentle stirring doesn't work, add hot water, a tablespoon at a time, while the mixture cooks. Add no more than a total ¼ cup (4 tablespoons) to recipe calling for 1 cup (2 sticks) of butter. Add water slowly and carefully as the water can cause the hot candy mixture to splatter. Adding the hot water lowers the temperature of the toffee mixture; therefore, continue to stir and cook the toffee until it reaches the correct temperature.