Chocolate is naturally quite brittle and will fragment under pressure. The grooved "teeth" on a serrated knife are like many tiny wedges pushing into the chocolate and getting it to fragment in several places at once. Also, serrated knives tend to be sharper than chef's knives since they're used less frequently and for less arduous tasks.
Start at one side or corner of the chocolate. Since the blade is so long, use just the part closest to the handle for maximum force. Use one hand to hold the handle and your other hand to exert pressure straight down (rather than side to side) to push the knife through the chocolate.
Work your way in toward the center a few millimeters at a time. The chocolate will fragment and flake away, giving you thin shards of chocolate perfect for melting or decorating.
If you need to chop your chocolate even more finely, we recommend switching back to your chef's knife. For larger fragments, just increase the width between each cut.
Next time you find yourself in need of some chopped chocolate, give this method a try and see if it makes a difference for you!
Related: Online Chocolate Source: Chocosphere
(Image: Emma Christensen for the Kitchn)