Preheat oven to 325 degrees F.
Line a baking sheet with foil, allowing an inch overlap on the ends. Spray well with cooking spray.
Place pretzels in single layer on baking sheet.
In a small, heavy sauce pan, melt the butter and sugar over medium heat. Bring to a boil and let boil for 3 minutes, stirring constantly.
Pour the toffee mixture over the pretzels. Don't worry about filling in all the spots - the toffee will even out in the oven.
Bake for 6-8 minutes, until the toffee mixture is bubbling.
Remove baking sheet from oven and sprinkle on the chocolate chips or chocolate candy melts evenly across the entire pan.
Pop back in the oven for a minute or two to soften the chocolate. Remove from oven and spread the melted chocolate over the entire pan using an offset spatula.
Put baking sheet in freezer for 30 minutes.
Remove bars and foil from pan. Remove foil from bars and cut into desired sizes.
Bars can be stored at room temperature in an airtight container, but we like to keep them frozen. They're great either way!