A number of people chimed in, letting me know that these were referred to as kale sprouts, lollypop kale, kale-brussels sprouts, flower sprouts, and petit vert (プチベール), in Japan. They're best cooked until the outsides are crisp, but tender inside. For those who like
kale chips, they could be cooked until completely crisp throughout, although they are messy to eat.
I can't say where you can get kale sprouts where you live, and I was surprised to find them in Paris! Trader Joe's in the U.S. sometimes carries them, but you may need to do a little searching at your local greenmarket, farmers' market, or well-stocked grocery store, to find them.
Variations include a sprinkling of Parmesan before or after roasting, a couple of cloves of finely chopped garlic, or red pepper flakes, to add a little heat. I've linked to some recipes and ideas after the basic recipe. Feel free to leave other ideas in the comments.