The first thing you're going to need is one of these bad boys
. Silicone half-sphere molds can run $10-$20 depending on where you get them from. If you're going to be baking up a lot of these cakes, it's better to have 2-3+ molds so that your cake batter isn't sitting around for an hour while you work through 10 baking cycles. (Batter left sitting around can kill your leaveners; it's best if you get your batter into the oven as soon as it's ready to go. Even more so when you're only using baking soda!)
There isn't one critical recipe for this project; any recipe that yields a firm cake with a dense crumb is perfect. Butter cakes work well because they get harder in the fridge. If you're up to it, make a vanilla and a red velvet batter like I did. If not, just make vanilla, separate half your batter and tint it with a bit of red food colouring. (This goes for making other types of pokeballs too, to get your green, blue, etc.)
You CANNOT use cake-mix for this.
I think cake-mix is a great way for people to get reliable, fool-proof light and fluffy cakes, but they are just too
light and fluffy for this. The cakes are small and get handled quite a bit during this process, and cake mixes just fall apart. I made this mistake myself: I decided to test out a new brand of cake-mix for my red velvet. The first batch fell to pieces when I tried to take them out of the mold. The second batch survived with ludicrous amounts of greasing the mold, but were very very difficult to work with when covering with fondant. So if you want to save yourself a ton of hassle, don't bother using it.
Follow the steps of the recipe that you choose, adjusting the bake time accordingly--much much less. I found mine were baking in about 8-10 minutes. Let them cool fully.