Chop the candy bars into large chunks. Spray a 9x13" pan with cooking spray. Preheat your oven to 350°.
Melt butter in a heavy small saucepan over medium heat. Whisk in the cocoa powder and sugar. Cook and stir for a minute or two until the sugar starts to dissolve.
Pour the chocolate mixture into a large bowl, let cool for 8-10 minutes, stirring often to speed up the process.
Whisk in the eggs and vanilla extract.
Stir in the flour and salt, then fold in the chopped toffee bars and about one cup of the toffee bits. (Save the rest for the top of the brownies.)
Spread the brownie batter in the prepared pan; sprinkle the remaining toffee bits on top.
Bake brownies at 350° for about 35 minutes, or until a toothpick inserted in the center comes out with moist crumbs.
Place the pan on a wire rack to speed up cooling. We love the brownies warm, but you will get a more clean cut if you let them cool first.
Notes
Make sure the chocolate mixture is cool enough to touch before adding the eggs - you don't want them to scramble!
An 8-ounce bag of toffee bits doesn't have quite enough, so I like to buy two bags. But you can skimp and they will still be tasty.
You can use Skor bars instead of Heath bars if you prefer.
For fudgy brownies, you don't want to overbake them. You don't want batter on the toothpick, but you want the brownies to barely be set in the middle.