I designed a simple illustration in Adobe Illustrator, saved it in SVG format, and then used a laser cutter to etch the image into a 3"x2", 1/4" thick piece of acrylic. Be sure to wash the piece with cold water to remove dust particles prior to next step.
Alternatively you could use wood. I like acrylic because it is clean after being etched and doesn't need any additional modifications such as sealing. Also, I would NOT recommend using masonite or similar material that can be cut with a laser if particles might find their way into your mold.
If you are interested casting multiple piece at once, I recommend designing a gang mold
at this stage.
Obviously this form is modeled after a Hershey's bar. If you are interested in making a grid of pieces that can be broken up, design your illustration for the form accordingly.