In heavy saucepan, combine butter, sugar, water, and corn syrup. Cook over medium heat to hard-crack stage (306°F) stirring constantly and watching carefully. Remove from heat. Add vanilla, stir.

Immediately pour into ungreased 13x9 pan. Cool couple minutes.

Sprinkle chocolate evenly over toffee. Spread with offset spatula. Sprinkle with nuts, pressing them into chocolate.

Let stand 2-3 hours or chill 30 minutes. Break into bite-size pieces.

Problem with butter separating: #1: Butter and sugar melting unevenly. Sometimes separation can occur in the beginning stages of toffee making, when the butter and sugar are melting. If they melt unevenly, separation might occur. Solution: Don't put the butter right from the refrigerator into the pan. Soften it slightly, either by leaving it on the counter for an hour or so before making the candy, or by placing it in the microwave. Microwaves vary greatly, but I have found that warming the butter on power 1 or 2 for 10 seconds, repeating as necessary until the butter is soft to the touch but not melted, works well. Also, be careful not to let the mixture boil until the sugar has dissolved.

#2: Perhaps water evaporating too quickly. In the beginning stages of making the toffee, you can get separation if you heat the mixture too high too quickly. Not only will the high heat affect how evenly the butter and sugar melt together, but the high heat can also cause the water to evaporate too quickly. Solution: Keep your burner to a medium-low heat in the beginning stages of melting, especially if you have a very effective range. And be sure to stir slowly, not too fast. Yes, it might take longer to cook, but the results will be worth it in the end. Nothing is worse than spending time and money on a homemade treat, only to discover the recipe didn't turn out!

#3: Temperature shifting abruptly. If you get impatient while waiting for the mixture to boil and turn the heat to high, you can "shock" your toffee, causing separation. Solution: Don't turn the heat past medium at any stage of the toffee-making process. Keep the heat constant. It will boil; just be patient.

Trouble-shooting while cooking: If you've taken all the precautions listed above and find that your toffee is still separating while you're cooking, all is not lost. You can sometimes save the mixture by removing the saucepan from the stove, slowly stirring the sugar and butter until the mixture comes back together, then returning it to the stovetop and continue cooking until it reaches the hardball stage. You can also add a bit of hot water, one tablespoon at a time, not to exceed ?¼ cup of water (4 Tbs.) per cup (two sticks) of butter. Adding water can lower the temperature and restore moisture that may have evaporated too quickly. If you add water, stir slowly, and be very careful, since the water can splash out of the pan and burn you.

Unfortunately, if you notice the separation when you pour the toffee onto the cookie sheet, it's too late to save, at least for your toffee recipe. Pour or wipe off the excess fat, let it cool, then crunch it up and use it as a hard topping for ice cream or other treats. (After all, it's pure sugar and butter, so it's bound to taste good, even if it isn't pretty!)