(spoiler for puzzle):
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1. go through the top right exit, then take the path to the right and keep going until you see a hole in the wall to get the cupcake
2. return to the previous map and go to the left. face the hole and use the cupcake
3. go through the top right exit again and this time walk up the stairs to the inscription, which you can write on now
you can interact with them in any order, the main thing is just to get the cupcake and use it to feed the rats first before you can proceed
i hope that was clear >-<