How to melt chocolate for hot coco?

i recently bought a k-cup brewer, and with it i got milk chocolate coco, and dark chocolate coco, ive tried both flavors and thought i could increase the taste by adding actual chocolate, so i bought 2 large Hershey's bars one milk chocolate and one dark chocolate, i broke off a single piece and put it in the cup then let the hot coco mix melt it, however this doesn't seem to work the result being a large mound of semi-solid chocolate at the bottom of the cup, i thought if i cut/broke up the piece of chocolate into tiny pieces that the additional surface area would allow the heat to melt the chocolate better, but this didn't seem to work ether? so my question is how do i add more chocolate flavor to my hot coco k-cups?