Why does toffee separate?

I’ve never had toffee fail, until today. I’d also never had caramels fail until 3 years ago. Both separated, just as you explained. I read the comments carefully, and tried again. Nope, another failure. The only consistent change was from my moms old pressure cooker pan (early 50s), to a new All-Clad d5 pan 3 years ago, and I’d tried a silicone spoon to stir.
The old antique pan finally broke, so the All-Clad just failed again with a wood spoon. I’m now switching to a Scan Pan. I’ll see how this batch goes, ugh.

You can rescue it, too. Just add water and sugar and start over.

Hi I am sorry to hear that about your toffee, I know it can be frustrating. The biggest reason that happens is the gas, or hear source, is too high. Place the heat at about 70% and try it that way, also it could be the ration of fat to sugar. Try equal parts butter and sugar ( one pound each and 3/4 teaspoon of salt) and cook it on medium high heat until nice and brown, smoking slightly. Pour on a non stick mat and score when it has cooled a little. Dip in untempered(melted and cooled) milk chocolate and cover in chopped toasted almonds. Sounds like you are just a little rusty, practice makes perfect and you need to nail this as the holidays are rapidly approaching! JK, I hope this helps!
Andrew

Nomnomnom, make sure that your heat is not too high either. It too can also cause the butter to separate out because it causes the water to evaporate too quickly from the mixture. Here's a good website for troubleshooting toffee:
http://baking911.com/learn/baked-goods/candy/types/toffee

I have made 25 pounds of toffee this week. I let the mixture reach 240 degrees before I stir, this stir continually to 295.

Might have stirred too much. I find that once the sugar dissolves and the mixture comes to a boil, I stop stirring. I know a lot of recipes says to stir constantly, but it's been found that this causes the the sugar mixture to separate. And that was my early experience with making butter crunch toffee. I couldn't understand why the butter was separating out. So on my 5th try I stopped stirring at ~200F, and left the sugar mixture boil to the soft crack stage, did not touch it at all, and voila, perfect toffee. And I make a toffee so good, my friends call it "crack" ;)