The correct pot for toffee making?

Stainless steel but be aware that the temperature will be much different than if you were using the cast iron. the stainless steel will heat up faster and not retain the heat as long as the cast iron. Just make sure you keep a close eye on your candy thermometer.

If you want to be pedantic the only correct pan for any type of sugar work is a copper pan. However most people don't have a call for a copper pot. I use stainless still pots all the time commercially. Make sure to have on a gentle heat. Use a pastry brush dipped in water to brush down the insides of the pan so nothing dries out on sides, then drops into your toffee to ruin it.

Remember the easist way to clean a toffee pot is to fill with water and bring to the boil.

Have fun

I do it in stainless steel every time. Go ahead.

Nikki P is absolutely correct.