Fantastic game! I really like the mechanic, and the levels are really well designed. I haven't tried the bonus levels yet, but I'm impressed you found time to make them.
Having the potions as an optional challenge was a really good idea. It's really satisfying to get something and know that you exceeded expectations.
The platforming controls are the only real complaint I have. They're functional, but a little odd at times. For example, when trying to jump onto a block that was moving towards me, the character would occasionally double-jump and go flying over the top.
I love the aesthetic, and having the player exit into the fresh air in the final level was a really nice touch.