Preheat oven to 325°F (160°C). Line a 15x10 inch baking sheet with parchment paper.
In a medium saucepan, melt butter over medium heat. Add sugar and salt. Bring to a boil, stirring constantly.
Continue to cook, stirring constantly, until the mixture turns a light amber color and reaches 300°F (149°C) on a candy thermometer. This should take about 5-8 minutes.
Pour the hot mixture onto the prepared baking sheet. Spread evenly with a spatula.
Bake for 5 minutes, or until the toffee is bubbly and evenly browned.
Remove from oven and immediately sprinkle with chocolate chips. Let sit for 2-3 minutes until the chocolate is melted.
Spread the melted chocolate evenly over the toffee. Sprinkle with chopped pecans.
Let cool completely. Break into pieces and store in an airtight container.