- 1/2 cup (1 stick) unsalted butter
- 1 1/4 cups granulated sugar
- 1 tablespoon light corn syrup
- 1/4 teaspoon salt
- 1/4 teaspoon baking soda
- 1/2 teaspoon vanilla extract
- 1 1/2 cups crushed thin pretzel twists
- 1 cup semisweet chocolate chips
- 1 teaspoon fleur de sel or other flaky sea salt
Line baking sheet with parchment paper. Set aside. Combine butter, sugar, corn syrup, salt, and two tablespoons water in medium-sized heavy saucepan. Clip candy thermometer to side of saucepan. Cook over medium heat, stirring as little as possible, until thermometer reaches 300ºF (hard crack stage), about 10 minutes.
Remove saucepan from heat and immediately stir in baking soda and vanilla extract. Stir in crushed pretzels. Quickly pour toffee onto prepared baking sheet, spreading it as thinly and evenly as possible.
Let toffee cool slightly until it just begins to harden, about 3 minutes. Sprinkle toffee evenly with chocolate chips. Let it stand until chocolate has melted, about two minutes. Using a spatula, spread chocolate evenly over toffee. Sprinkle chocolate with fleur de sel.
Transfer baking sheet to refrigerator until chocolate has hardened, about 1 hour. Break toffee into large shards. Store in airtight container on countertop or in refrigerator for up to three weeks.