How to the Cake/Pie minidungeon?

I've seen a couple of screenshots this year of the Cake/Pie mini-dungeon, not to mention their appearance on Fiel's extractions. However, I tried last year's Sphinx portal a couple of days ago, and it doesn't work. How do you get to the Cake/Pie mini-dungeon?

The portal is between the Pie Lord and the Cake Boss. You have to be on the winning team to enter.