There are specialty tools called chocolate chippers that can be purchased and used to break up chocolate, but a chef’s knife or a serrated knife works just as well. Choose a sharp, heavy chef’s knife (a large straight-bladed knife, usually 8-10 inches) and press down firmly and evenly on the chocolate, beginning with the corners and angling the knife slightly outward. Whittle the chocolate gradually, working from the corners, until the chocolate is chopped into almond-sized pieces.
A long serrated knife also works for chopping chocolate, and it requires less force to be effective. Again, begin at a corner of the chocolate and use a smooth sawing motion back and forth, pressing only as hard as necessary. Once you have made several cuts on a particular corner, rotate the chocolate and begin on a new corner until all of the chocolate is chopped into uniform pieces.