01 - Preheat oven to 300°F. Line a large baking sheet with parchment paper.
02 - Place the pretzels in a large mixing bowl.
03 - In a saucepan over medium heat, melt the butter. Stir in the brown sugar, corn syrup, and sea salt. Bring to a gentle boil, stirring frequently, and simmer for 2–3 minutes until smooth and bubbling.
04 - Remove from heat and stir in the vanilla extract.
05 - Quickly pour the hot toffee mixture over the pretzels. Use a spatula to gently toss until all pretzels are evenly coated.
06 - Spread the coated pretzels in a single layer on the prepared baking sheet.
07 - Bake for 12–15 minutes, gently stirring halfway through, until the toffee is set and bubbly.
08 - Remove from the oven and immediately sprinkle with flaky sea salt, if using. Allow pretzels to cool completely on the baking sheet. Once cooled, break apart any clusters and transfer to an airtight container.