Grease the bottom and sides of a rimmed baking sheet with butter and set it aside.

In a large heavy-bottomed pot, melt the butter with the sugar, salt, and vanilla over medium-low heat, stirring frequently.

Let the mixture come to a gentle simmer and begin to thicken, continuing to stir often.

Cook until a candy thermometer inserted into the mixture reads 260 degrees F.

Stir in the pecan pieces and mix well to coat them completely.

Continue cooking over medium-low heat, stirring constantly, as the mixture thickens and turns a deeper caramel color. This will take about 15 to 18 minutes.

Watch for the mixture to bubble and darken, and stir consistently to keep the toffee from burning on the bottom of the pot.

Once the toffee reaches 295 degrees F on the candy thermometer, carefully remove the pot from the heat and immediately stir in the baking soda.

Stir vigorously; the toffee will puff up as the baking soda aerates the mixture.

Quickly pour the hot toffee onto the prepared baking sheet, being careful not to touch it because it is extremely hot.

Use an offset spatula to smooth the top of the toffee into an even layer.

Sprinkle the chocolate chips evenly over the hot toffee. Let them melt, then spread the chocolate into a smooth layer with an offset spatula or the back of a spoon.

Dust the top with the chopped pecans.

Cool completely at room temperature until the toffee and chocolate are fully set, or place the pan in the refrigerator to speed up the process.

Break the toffee into pieces with your hands and serve chilled or at room temperature.