Double Chocolate Toffee Cookies are made with bittersweet chocolate chips and chopped up Heath toffee bars. The original version, Giant Chocolate Toffee Cookies from Bon Appetit, called for regular chocolate, but over the years I've found that regular chocolate chips work well, too. I also make mine a bit smaller.
Bittersweet (60%) worked well, but you could even go with a 70% chocolate bar or something even less sweet.
The cookies spread as they baked, the puffed up towards the end. The flattened out a little bit upon cooling.