Start by gathering all your ingredients. Line your baking dish with parchment paper. Spread the chopped pecans evenly at the bottom to form a crunchy base.
In a saucepan, combine butter, sugar, water, and salt. Heat over medium while stirring until it melts and begins to bubble. Attach a candy thermometer and aim for 300°F (hard crack stage).
Keep stirring occasionally as the mixture heats. Once it reaches 300°F, remove from heat and stir in vanilla extract for a smooth, aromatic flavor.
Pour the hot caramel over the nut layer in your baking dish. Spread evenly with a spatula for uniform thickness.
Sprinkle chocolate chips over the warm caramel and cover briefly to melt them. Swirl the chocolate gently, then top with extra chopped nuts and a pinch of flaked sea salt if desired.
Allow the toffee to cool completely for about 4 hours. You can refrigerate to speed up the process. Once set, remove from the pan and cut into pieces or break by hand.