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