Preheat oven to 350°F. Line a half-sheet pan (or 9x9-inch square pan) with foil. Spray lightly with cooking spray.
Melt the butter and unsweetened chocolate in a small saucepan. Transfer the melted mixture to a bowl and let cool slightly. Whisk in the sugar, flour, and eggs. Stir in the chocolate chips, toffee bits, and pretzels.
Pour the mixture into the prepared pan and spread evenly (the batter will be thick and chunky). Bake 12 to 15 minutes, or until the brownies appear firm and done.
*A half-sheet pan will give you thinner brownies. A square pan will result in a taller brownie.
*These are best when eaten within 24 hours of coming out of the oven.