01 -
Line a 9 x 9-inch square baking dish with parchment paper. Distribute 1 cup chopped pecans evenly on the bottom.
02 -
Combine butter, granulated sugar, water, and salt in a heavy-bottomed saucepan. Place over medium heat, stirring until butter is melted and the mixture is bubbling.
03 -
Attach a candy thermometer to the pan and continue heating, without stirring, until the mixture reaches 300°F (hard crack stage).
04 -
Remove the pan from heat and immediately stir in vanilla extract. Carefully pour the hot toffee over the chopped pecans in the baking dish, spreading out evenly.
05 -
Allow the toffee to rest for 2 minutes. Sprinkle chocolate chips evenly over the surface and cover the dish for 4 minutes to let the chocolate melt.
06 -
Uncover and gently swirl the melted chocolate with a knife or spatula. Top with the additional chopped pecans and flaked sea salt if desired.
07 -
Allow toffee to cool completely at room temperature for 4 hours, or refrigerate to speed up the process. Once set, lift the slab out using the parchment, then cut or break into pieces.