The game is full of hard levels. A lot of levels have very few moves, i.e. 25 moves or less. My idea is that if you get a golden crown in any hard level (passing it in the 1st try), you'll get gold bars.
The rewards would be:
Normal (purple button) hard level: 1 Gold Bar
Super hard level (dark blue button): 3 Gold Bars.
Nightmarishly hard level (black button): 5 Gold Bars
Legendary level (black button with flames): 15 Gold Bars