From IFAB Laws of the Game, Law 14, part 2,
... play will be stopped and restarted with an indirect free kick, regardless of whether or not a goal is scored:
feinting to kick the ball once the kicker has completed the run-up (feinting in the run-up is permitted); the referee cautions the kicker
Thus, feinting during the run-up is permitted, but feinting once the player has completed the run-up is not.
In the latter case, the kicker should be cautioned. From 1st June 2016, the defending team will receive an indirect free kick even if the ball enters the goal. Previously, if the ball entered the goal, a retake would occur after the caution was issued.
From IFAB Laws of the Game, Law 14, FAQ:
Q2: Why is illegal feinting by the penalty kicker an IDFK even when the player scores?
A player who deliberately stops at the end of their run and then feints to gain an advantage is deliberately breaking the Law. This is an act of deliberate unsporting behaviour so, as well as the caution (YC), the player does not deserve to have a second chance to score. This stronger punishment should deter an offence which is sometimes difficult to detect.