Here is the full list of all 16 Monsters Ate My Birthday Cake achievements.
Eat 100 pieces of cake
Collect 5 costumes
Open 25 treasure chests
Find all the monster companions
Use Poot to toot on a Boogin
Freeze a Boogin into a block
Get 5 'Spare all Boogins' challenges
Free Lil' Willis 10 times
Collect 150 Stars
Find All Message Bottles
Complete all of Poe's requests
Complete Nogport Meadows
Complete Powder Peaks
Complete Apricot Acres
Complete The Deadlands
End the Boogin King's reign