graduated circle cutters, but you could use anything at all. Before I got these cutters, I used to use the lids off my spice jars for circles. They come in all sorts of sizes. (And don't you be bothered by the weird cracks in these cookies. I messed up my first picture and had to retake it. Which means I had to re-bake some cookies. And all I had left was the weirdy cookie dough that I was about to throw away.)
I should have been using food markers the whole time.
If you use a knife to spread the white icing on these guys, you wouldn't have to pipe a thing. Then you could use the time you would have spent filling icing bags/bottles to do some more research.