either print one off or use the principles of geometry and a pair of compasses and protractor (sp?) to draw one, then use it as a template and cut them out of black fondant icing, place on top of the white-iced cake :-) Simples!
I have seen hexagonal cutters (used fipor scones I think) from baking shops or ebay. Maybe you could cut the hexagons out of rolled out fondant icing (you can get ready made in supermarkets if thast's easier. To make the actual cake, what about cooking it in a pudding basin in the microwave? (Microwave sponge cake recipe easy to find online- sorry am on phone)