Line a large rimmed baking sheet with parchment paper.
To a medium, heavy-bottom saucepan over medium heat, add butter and sugar. Cook, stirring consistently with a wooden spoon until the candy reaches 285°F on a candy thermometer. (It should be a dark amber color.) This will take between 35-45 minutes; don't rush it. If your temperature is rising too quickly, turn the heat down slightly.
Pour the candy mixture onto an 11x15 lined baking sheet. While still hot, top with chocolate chips in an even layer. Let it rest for 1-2 minutes to allow the chocolate time to soften and melt. Carefully spread the chocolate into an even layer.
Sprinkle with pecans and gently press them into the melted chocolate.
Let it sit for about 1 hour, or until set.
To serve, break into pieces.
To store, place pieces in an airtight container at room temperature for up to a week.