Yes, Brain Age is the most commonly used DS game for CFW, which you must buy from the eShop. There are 2 ways you can use it - you can either use it to install standard Haxchi, which requires you to launch the DS game after every reboot for CFW, or ColdBoot Haxchi, which uses the same DS game but launches it on startup. As KleinesSinchen mentioned, this can be dangerous, as moving or deleting the DS game before uninstalling CFW
will lead to a brick. I use CBHC personally, and it's safe as long as you can trust yourself not to move or delete it (the name of the DS game changes to 'DON'T TOUCH ME' to remind you). Again,
https://wiiu.hacks.guide/get-started will tell you how to set up either of them.