Salted butter in toffee

@Chris: Does the recipe from the Nero Wolfe cookbook say anything about dumping the milky/watery portion out of the pan before returning the butter to it to brown?

Clarified butter WILL NOT brown, that is the purpose for clarifying it. The milk solids are what brown. The portion that usually goes to the bottom will be the whey and the milk solids initially tend to form the "scum" on the top. To me it sounds like he's trying to suggest that you should pour the butter & solids off, dump out any whey, and then return the butter to the pan so you can heat it to the point of a dark brown without it splattering (which is caused by the water in the whey). 4 tablespoons of butter isn't going to have much whey in the first place so just cook the whole butter to the beurre noir point.

The higher the temperature, the harder the caramel. That is basically the whole story. So I think what is happening to you is that the caramel in the middle is still going up in temperature due to residual heat, while the stuff at the sides cools down quickly because it can vent heat through the pan to the outside world. Have you checked the calibration on your thermometer? In any case, I think the things you can try are (1) cook to a slightly lower final temperature (back down a couple degrees at a time) and (2) set the pan over a tray of ice to cool it more evenly. Also, be sure to do a really good job of that final stir before pouring into the tray.