Tortoise and the Hare (early access)

Play as both the tortoise in the hare in this single-player, double platformer! 

Words I would use to describe it include 'classic', 'difficult', 'hardcore', and possibly 'rage-inducing'... 

Oh, and brain-bending. A bit like rubbing your head and patting your tummy. 
But seriously, there's a point in playing where it just clicks, and suddenly you're addicted.