Gonna be honest I am not really good at UI haha normally when doing this type of art i would just use something like photoshop or similiar since i have a drawing tablet, so I have no idea what sort of apps are avaliable.
Personally i feel like small details make it much better for example adding some sort of movement to the buttons on the main menu, or when dropping an ingredient on the cauldron make some particles pretending to be the item dropping.
I also believe the main menu "logo" seems blurry?Might just be a sizing issue not sure
Those are minor things, but i feel like they would add even more to the already great concept!