The birthday girl and her entire family loved the Spinning Disco Ball Cake, and I felt very proud that I retired from making her birthday cakes on such a high note!
Spinning Disco Ball Cake - November 2010
first I harvested the spinning base of a plastic spice rack from a thriftstore which had a screw in the center.
I used PVC since it is easy to sanitize, so about a foot of 1" diameter thick-wall PVC pipe was the post up to the centerpoint of the ball, and the post fit into a PVC converter piece that I glued to the spinning black base, while the base was screwed to the baseboard. That way I could fit the PVC post into the motor assembly onsite after driving the entire cake 150 miles away. Radio Shack still had local storefronts back then, so I bought a battery case and a thick rubber belt, which worked pretty well for the motor to drive the center post. There wasn't much height on the motor post, so I added a dot of hot glue so the rubber belt wouldn't slip off the top while it was spinning. Since all this was inside the base box and only needed to work for one evening, I used wire nuts for my connections between the motor at the battery case, but I was proud that I thought to add a toggle switch to the outside of the box!
The bottom base was wood painted black for the screws for the spinning base and motor to anchor securely, then the rest of the base box was foamcore scraps since it was only cosmetic, with holes cut for the center post to set through the top piece into the spinning base, and the toggle switch embedded in the side wall. After assembly the entire box was painted black. I could have tried covering the base with a layer of cake, but just as well I didn't, since not even the whole disco ball was eaten at the party. ;)
...but how to make the actual ball, and how to make it reflect like real mirrors? Luckily I had one plastic mixing bowl that was very close to a true hemisphere, and the birthday girl preferred rice cereal treats over cake anyway, so there was no actual cake in this "cake" project. I made several batches of rice cereal treats, lined the bowl with plastic wrap, then filled each hemisphere completely and let it cool in shape before removing it to make the other half of the ball. I knew there was no way I would be able to transport a globe suspended on a post driving 150 miles, so I cut matching bases from cardboard, making sure the top cardboard center hole fit exactly around the PVC endcap so they nested together to avoid the top sliding off the bottom.
Once I knew the size of the finished sphere, I did a lot of geometry sketching to figure out what shapes and how many mirror tiles I needed. I knew from previous sugar glass work that you cannot just cut the sugar accurately once it has hardened, so I made a food-grade silicone mold from clay positives, requiring several sets of sugar 
glass filling the same mold. I couldn't pour the sugar completely accurately, so the messy edge bits thankfully were able to break off by hand or be trimmed carefully with kitchen shears.
After all the sugar glass tiles were trimmed and cooled, I painted all the backs with silver luster dust, just like real mirror tiles are silvered on the back. Of course this took a lot of time, and I also made sure I had extras in case of any problems in transit! Even though the sugar glass was slightly yellow, not perfectly clear, they still looked like they would actually reflect light like I hoped. I stacked the silvered tiles 
between wax paper in airtight containers, made some blue marshmallow fondant & lots of black royal icing, then all that got packed safely in clean plastic in a cardboard box for the 150-mile drive, to be continued 40 miles away from the final party destination.
For the base I made homemade marshmallow fondant tinted blue, with the center post and the 4 light covers covered with fondant to match. I used food coloring pens and silver luster dust for the birthday message on the blue base. I waited until after the 150-mile drive for the fondant work, since there was too much risk of marring the surface in transit. Once the blue fondant was on the base, I set everything up to aim the lights at the four corners so the fondant would set overnight with the base, and to test the spinning speed with the rice cereal weight. I didn't add the blue fondant covering the center post until 
after the mirror tiles were glued and dried overnight.
I worked on foil large enough to wrap around each hemisphere so I minimized how much I had to touch the mirror tiles to avoid fingerprints. As soon as mirror tiles started going on the black royal icing, it started reflecting just as planned...hooray! When I got to the poles it was a little trickier for the final tiles than my geometry 
sketches, but I was still very pleased with the result. It's always easier washing off the food coloring and cleaning up for some rest after a plan comes together successfully!
