Line a 17x12-inch jelly roll pan with parchment paper and set aside.
Combine the butter, sugar, vanilla extract, and salt in a large, heavy-bottomed saucepan over medium heat; attach a candy thermometer to the pan.
Stir until the butter and sugar melt and the mixture is smooth, then bring it to a boil, whisking constantly.
Continue whisking as the mixture boils until it reaches 290°F (hard crack stage), about 10–15 minutes; the color should be amber-caramel.
When it reaches the hard crack stage, immediately pour the hot toffee into the prepared pan and spread it evenly.
Let the toffee sit 1–2 minutes, then sprinkle the milk chocolate chips evenly over the hot surface.
After about 1 minute, when the chips have softened, spread the chocolate smoothly with an offset or rubber spatula and sprinkle the chopped pecans on top.
Let the toffee cool at room temperature until fully hardened, about 2 hours, then cut into pieces with a thick, sharp knife.