Ideally, both!! Let me explain:
For Genoise, you cook the butter until the solids are brown, “buerre noisette”, then pour off the butter and leave the solids behind (you can strain it if you like, but I just pour off the butter slowly, so the solids stay in the pan).
You can then scoop out the solids and freeze them for the next time you make bread. If I have enough toasted butter solids in the freezer, I put a teaspoon or two in brioche, bread, even chocolate chip cookies!
Let’s see, if you are making just clarified butter (not browned) as for Creme Ivoire, I think it’s best not to brown the solids, as this produces too much toasted flavor for that buttercream. In that case, you can still pour off the clarified butter when the solids drop, but then return the pan to the heat long enough to toast the solids and add them to your tasty freezer stash!
Have fun with TCB, what are you going to make first?