Can baking sheets continue to cook toffee? I make tons of toffee every year. And I mean tons. Never had a problem. Now this year, something so wrong. Same recipe. Same pans. Same house. Same stove. Same thermometers. I even bought new thermometers just in case. So here is the deal. Everything is fine. Cooks fine. Looks good. Smells great. Once it reaches desired temp, I pour it onto a lightly buttered baking sheet. Same as always. Cooks down. Put it in freezer. Everything is the same as I have done for YEARS!  Now when I take it out and break it up, it’s perfect around the edges but super dark and almost burnt in the middle. I can’t figure it out. So my question is this?  Could the baking sheet be “cooking” the mixture?  Can the pans go bad?  I don’t understand how only around the edges is good. I can’t stress enough how everything is the same down to the brand of ingredients. I’m about to lose my mind!  PLEASE HELP!
Toffee cannot continue to cook once it's been poured onto a baking sheet.  There is no heat source at this point and therefore there can be no increase in temperature.  

All other factors being equal, I might suggest looking at your ingredients.  Are you using beet sugar this year when you usually use cane sugar?  It might seem like a trivial difference, but beet sugar does caramelize differently from cane sugar, so it's a relevant concern for toffee-making.

However, you might not need to get to the bottom of this mystery.  You know that the temperature you're using with the current ingredients and equipment is burning the toffee.  You could try cooking the next patch to 5-10 degrees less than your previous attempts so that it won't burn.  I'm pretty sure that 290 degree toffee will be just as crunchy and lovely as 300 degree toffee.