Line a 17x12-inch jelly roll pan with parchment paper.
Combine butter, sugar, vanilla, and salt in a large, heavy-bottomed saucepan over medium heat. Clamp a candy thermometer to the saucepan.
Cook, stirring occasionally, until melted and smooth, then bring to a boil while whisking constantly.
Continue whisking the mixture as it boils until it reaches 290°F or the "hard crack" stage. This will take 10–15 minutes, and the mixture will be amber-caramel in color.
When it reaches the hard crack stage, immediately pour the toffee into the prepared pan.
Let sit for 1–2 minutes, then sprinkle chocolate chips on top. After 1 more minute, spread the chocolate smooth with an offset or rubber spatula and sprinkle chopped pecans on top.
Let sit at room temperature for 2 hours to harden, then cut into pieces with a thick, sharp knife.