After you break into the Chapel and stop Booster's wedding, the angry chefs confront you with Bundt.
Battle the Tortes and Bundt
The Tortes can't be damaged, so just keep hitting Bundt. Eventually, the Tortes will run away.
At this point, the candles on Bundt's head will light up, and you have to keep hitting it with regular attacks. Bundt will light up one or two of its candles on its turn, so try to keep everyone healed up so you don't miss a chance to blow out the candles.
Bundt can hit with a regular attack, which you can block if you press the button when Bundt's mouth opens wide and its middle layer is pulled back. It can also launch an orb at a character, which can be blocked. Bundt can attack Lulla-Bye, which inflicts sleep on a random character; Blizzard, which does a small amount of damage to all characters; Sand Storm, which reduces your attack power; Drain Beam, which does high damage to all characters; and Diamond Saw, which does very high damage to one character.
Eventually, the top part of the cake will disappear, leaving only the lower layer (Raspberry). It can still use the same attacks as before, but it can't regenerate its health, so just keep attacking until there is a cutscene.
After the Battle
Afterward, exit the room, save at the Save Block, then exit the room. There will be a cutscene. You can now go out through the front doors of the chapel.
When you're ready to move on, leave town. You will automatically go back to the Mushroom Kingdom. Go into the castle and there will be a cutscene. Go to the throne room and there will be a cutscene.
Next, exit the castle and there will be a cutscene. Go west to where you saw the Princess, and talk to her. After the cutscene, exit the Mushroom Kingdom and go to Tadpole Pond.
Go northeast and step on the rock, then jump across the tadpoles to reach Frogfucius. Talk to him and he will tell you to check Star Hill. Exit Tadpole Pond and make your way southwest toward Moleville, then go west to reach Star Hill.