2 tsp. plus 2 cups butter
2 cups sugar
2 tbsp. light corn syrup
2 tsp. kosher salt
2 cups chopped toasted pecans
1. Grease the bottom and sides of a 10"×15" jelly roll pan with 2 tsp. butter and set aside. Put remaining butter, sugar, corn syrup, salt, and 1⁄2 cup water into a medium pot and bring to a boil over medium-high heat while stirring constantly. Attach a candy thermometer to inside edge of pot and cook, continuing to stir constantly, until sugar mixture is deep golden brown and registers 310° (the hard-crack stage; see Cooking Sugar), about 20 minutes.
2. Pour the hot toffee onto the prepared pan and, using oven mitts, tilt and turn the pan to fill it evenly. Sprinkle the pecans over the top. (If you'd like to coat the toffee with chocolate before adding the nuts, let the toffee cool for 5 minutes, then sprinkle 2 cups semisweet chocolate chips over the top. Let sit for 1 minute, then use a rubber spatula to gently spread the melted chocolate evenly over the top. Sprinkle the pecans over the chocolate and gently press them down.)
3. Refrigerate toffee until it hardens, about 1 hour, then break into bite-size pieces. Serve toffee immediately or store in an airtight container in the refrigerator for up to 2 weeks.
Makes About 2 1⁄2 Pounds