"This cake is too sweet!"

Translation: 这个蛋糕太甜了！

[VWieVendetta]

Where's the 了come in? Is it that it's become too sweet in your mind or something?

[NasuSamaruk0]

Part of grammar, involving the combination of 太 and 了. The sentence expresses extremity of sweetness.

[Erikku15]

Whenever you have 太 you need to add 了 after 例如 太高了 too tall 太遠了 too far 等等