Preheat your oven to 400 degrees Fahrenheit. Line a 10×15×1 inch baking sheet with aluminum foil and spray with pan spray.
In a medium saucepan, combine 1 cup unsalted butter, 1 cup packed dark brown sugar, and ¼ teaspoon salt. Set it over medium-high heat and whisk the mixture together. Bring it to a boil, and allow it to bubble for about 4-5 minutes until it forms a luscious toffee.
Stir in 2 cups crushed pretzels until they’re well coated in the toffee mixture. Pour the mixture onto the prepared baking sheet, spreading it out as evenly as possible. Bake for 5 minutes.
Remove the baking sheet from the oven and sprinkle 2 cups semi-sweet chocolate chips evenly over the hot toffee. Let it sit for about 2 minutes, allowing the chocolate to melt.
Spread the melted chocolate over the toffee, then sprinkle with flaky sea salt. Let it cool completely before breaking it into pieces.
