This sort of thing happens at my house quite a bit: I made a cake to use up some egg whites. Or similar situations arise, at least.
Let me explain: I whipped meringue the other day but had more than I needed and rather than bake the extra I put it in the fridge. Then a few days later I remembered it and baked a chocolate cake so I could mix crushed candy cane into the meringue, pipe a disk and bake that too.
Then I assembled two chocolate cake layers with a little chocolate buttercream (also found in the fridge) between the cakes, none on the sides, and placed the disk on top, pressing the edges over, creating a sort of toadstool-shaped topping.
Because I had some whipped egg white I didn’t want to waste.
Does this happen at your house, too?
To complete the picture a little more, let me say the meringue in the fridge had separated, so there was some softish whipped egg whites sitting atop some liquid, murky egg whites in a large glass measuring jug.
I used electric beaters to bring it back together, then continued beating for a few solid minutes until it began to stiffen up. Probably 5 minutes with the mixer; if I’d used a whisk I would still be at it, rather than reporting back to you now.
So if this happens, and you have separated meringue, just keep beating and it will stiffen back up with time. More time than you think, because it seems like nothing is happening, but it will get there.
Then, like me, you can top a freshly baked layer cake with the meringue disk you made and there will be no waste.
Or, possibly, you created more work. Either way.