Michael Olise had a shot saved in under two minutes as the hosts – who have dropped points in one of their opening three league games for the first time since 2021 – made a fast start.
It was not long before they broke the deadlock, with Jamal Musiala firing home a 20-yard screamer. The German international received possession just outside the area, shifted the ball onto his right foot, and unleashed a vicious shot that pierced through two defenders and into the far corner.
It was nearly 2-0 shortly afterwards, only for Jonathan Tah's powerful diving header to be kept out by a superb save by Frederik Ronnow, who sprang to his right and got a strong arm on the ball.
Olise was then denied again, before Manuel Neuer turned sweeper-keeper at the other end, racing out of his goal to make a vital sliding tackle on Emmanuel Latte Lath.
While that challenge was timed perfectly, the same could not be said for Felix Uduokhai, who tripped Olise as he danced into the area. A penalty was duly awarded, and it was converted by Kane, who sent Ronnow the wrong way.
Olise then made it three before the break, clinically whipping into the top corner from outside the area.
The visitors looked to hit back after the break, and only a stunning save from Neuer prevented Janik Haberer from pulling one back, while the goalkeeper also punched a Tom Rothe header over the bar.
Bayern proved more clinical when the chances came their way, with Ballon d'Or favourite Kane nodding home his second from an in-swinging Olise cross.
Ronnow then twice denied Luis Diaz as the onslaught continued, but he was powerless to prevent Ismael Saibari bagging his first goal for Vincent Kompany's side. The Morocco international, a summer signing from PSV, dipped his shoulder to burst past three men before firing past the goalkeeper.
Things continued to go from bad to worse for Union, as Olise beat the offside trap to race onto a ball over the top and smash home via the post.
The Frenchman then wrapped up his first hat-trick for the club in spectacular fashion, unleashing a wonderful strike that curled over the defence and dipped into the net.
That was the 17th goal Mauro Lustrinelli's men have conceded this term, a high for the division, and set up their heaviest ever defeat at this level.
Meanwhile, Bayern move to the top of the table with this win, at least for the time being, matching their largest winning margin of 2026.
Flashscore Man of the Match: Michael Olise (Bayern Munich)