In a large bowl whisk together melted butter, granulated sugar, and brown sugar. Whisk in the egg and vanilla extract. Add flour mixture and fold, using a rubber spatula or wooden spoon, just until combined. Do not over mix. Fold in chocolate and pecans.
Spread dough evenly into prepared pan and smooth the top. Bake for 22-25 minutes, until the top is lightly golden brown and slightly firm to touch, and the edges start to pull away from the sides of the pan. The bars should appear soft but will set once cooled; over baking will make them dry and cakey. Allow to cool to room temperature on a wire rack, then lift out of the pan and cut into squares. Serve warm or at room temperature.
Store bars in an airtight container at room temperature for up to 2 days, or freeze for up to 2 months. Thaw at room temperature or overnight in the refrigerator.