Before we start anything, this glitch or bug has not been fixed yet! I am not the founder of this glitch and do not take any credit of finding it! I do still break games in a way that it makes it interesting to read my findings.

The cake is a lie.. The cake is a lie.. The cake is a lie!! or is it...?
Although the cake isn't visible till you beat the game, what if i told you can get there without beating the game? With noclip of course it's possible.. but legitly! There's an actual way to get to the "Cake Room" without using any type of cheats, hacks or the developer console. You get to see the cake and your best friend! What're we waiting for?! Lets go!

• Step 1
  We need to go to the Chapter 11 After the Turret Showdown.

• Step 2
  Go to this wall here next to the door leading to GLaDOS

• Step 3
  When Here, you want to place your two portals on the same wall. This is what we call a portal
  bump.

• Step 4
  If done correctly you have just made a portal bump and now have access outside the map!
  What you want to do next is to slowly move your way out of the portal and to look Straight up
  and a bit to right to where i'm aiming.

• Step 5
  Now shoot your portal then quickly move back before your portal hits surface of the wall you
  you just shot. If done correctly you should see the "outside" from the final scene.

• Step 6
  Now if you see the "outside" do the same as you done on Step 4 and aim towards this room
  here towards the bottom left.

• Step 7
  Now shot the same portal you've used on step 4 and quickly move back before the portal hits
  the surface of the wall. if done correctly, you should see this or a simulatory!

• Step 8
  If you done every thing correctly, you can now enter the portal completely without getting
  stuck!(if you do get stuck, just shoot a portal below you or near you so you can unclip yourself)
  Now if we see here, there's the cake from the final scene!

• Step 9
  Now that we're on top of the cake room, we need to glitch into the room! to do so, you need to
  come to this wall here and shoot one portal on the wall and one on the flooring like so

• Step 10
  This one takes a lot of patience to do cause it may take you awhile to succesfully glitch into the
  room. Make sure to save cause you can die from this! the best method to do this, (from hand
  on experience) is to hold crouch and move your mouse from left to right. If done correctly you
  will glitch through the ceiling into this spot here!

• Step 11
  You can now freely move around in the cake room! Congratulations!! You get to finally see the
  cake and your best friend, "companion cube"!

• Step 1
  Grab the companion cube and place it on top of the cake, like so.

• Step 2
  Now it's time to retrace our steps! Use one of the portals you used to glitch into the room so you
  can leave!

• Step 3
  portal your self back by retracing your steps and using the same technique so you don't get
  trapped! make sure to save so you don't need restart again! When going back try to get your self
  above the room you first made your portal bump and glitch into the room, like so.

• Step 4
  When done correctly, you should be back where you first made your portal bump! Go into GLaDOS's chamber and beat the game!

• Step 5
  If you done everything correctly! the cube should still be in the original spot you left it!
  (On the cake) So now wait for everything to go as normal.. as the claw goes to blow out the
  Candle it can't cause the cube is in the way! So that basically means that you broke the game
  into going to the credits! Good job!