1. 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.
2. Burning Toffee: The color of butter toffee should be a rich golden amber color. The toffee continues to change color and becomes darker as the temperature rises. If toffee cooks to too high a temperature and the toffee is dark in color, unfortunately, there is no way to save this batch of toffee.
Ways to prevent this from happening include:
If you use a Candy Thermometer test it for accuracy.
Use medium heat.
Use correct size of heavy-gauge saucepan.
3. Crystallization: One of the greatest frustrations in toffee making comes when a smooth syrup turns quickly into a grainy mass. This is caused by sugar crystals that have formed on the sides of the pan in the process of being stirred down into the syrup.
Here are several ways to prevent crystallization:
Dissolve sugar completely before mixture boils.
If you notice any crystals on the side of the pan, brush them down into the syrup with a pastry brush dipped into hot water or tightly cover the saucepan and let the mixture cook for about 3 minutes. This causes steam, thereby melting the sugar crystals that may have adhered to the sides of the pan.
Avoid stirring syrup once it begins to boil unless the recipe instructs otherwise.