Line a large rimmed baking sheet with parchment paper.
In a medium heavy-bottom saucepan over medium heat, add butter and sugar. Cook, stirring consistently until the candy reaches 285°F on a candy thermometer. This takes about 35-45 minutes; it should be a dark amber color.
Pour the candy mixture onto the prepared baking sheet. While still hot, top with chocolate chips and let sit for 1-2 minutes. Carefully spread the melted chocolate evenly over the toffee.
Sprinkle the chopped pecans over the melted chocolate and gently press them in.
Allow the toffee to cool and set for about 1 hour.
Break into pieces and serve or store in an airtight container at room temperature.