I'm sorry I don't have a foolproof recipe for penuche, but I can offer some common reasons why candy fails.

More than any other kind of recipe, candy recipes are notoriously unforgiving. Substitution of ingredients or the slightest deviation in technique will cause major trouble. It's said that baking is "science"; candymaking is even more so.

Humidity is a candymaker's worst enemy. If you're trying to cook candy on a humid day, you're almost bound to fail. Be sure to use your air conditioner, or else limit candymaking to the coldest, driest days of winter. I have heard that the sugar itself can hold extra moisture, just like flour can, and that drying it out with the use of a food-safe antidessicant before cooking can help ensure a successful outcome. I'm sorry, I don't know the details of that process, but it certainly makes scientific sense.

The penuche recipes I've seen require the mixture to be cooked to 237, not 230 degrees F. That might be your problem right there.

Be sure your candy thermometer is accurate. Check it by bringing a pot of water to a rapid boil; dip the thermometer into it. If the temperature does not read 212°, you'll have to either buy a new one or adjust your recipe to allow for the difference.

Finally, for what it's worth, I've been cooking and baking for over 35 years, but success at candymaking continues to allude me. It really is harder than you'd think, so don't feel bad about a few failures here and there.