Prepare Baking Sheet: Line a 9x13 inch baking sheet with parchment paper, leaving some overhang to easily lift the toffee later.
Cook Butter Mixture: In a medium saucepan over medium heat, combine the unsalted butter, granulated sugar, water, and salt. Stir until the butter melts and the sugar dissolves.
Boil and Stir: Bring the mixture to a boil, stirring constantly. Use a candy thermometer and cook the mixture until it reaches hard crack stage, about 300°F (150°C). This usually takes about 10-15 minutes.
Add Vanilla: Once it reaches the desired temperature, remove the pan from heat and stir in the vanilla extract carefully.
Spread on Baking Sheet: Pour the toffee mixture onto the prepared baking sheet and quickly spread it into an even layer using a spatula.
Add Chocolate Chips: Immediately sprinkle the semi-sweet chocolate chips over the hot toffee. Let them sit for about 2-3 minutes to soften, then spread the melted chocolate evenly across the surface.
Top with Pecans: Sprinkle the chopped pecans generously over the chocolate layer, pressing them down slightly.
Season with Sea Salt: Finish by sprinkling a light dusting of sea salt on top for added flavor.
Cool and Set: Allow the toffee to cool at room temperature for about 1 hour. Once set, refrigerate for an additional 30 minutes for an easier break.
Break and Serve: Once fully set, lift the toffee out of the pan using the parchment overhang. Break it into pieces and serve.