At once and Piece of cake

At once adverb - Without delay or hesitation; with no time intervening.

Piece of cake and at once are semantically related. in quickly topic. In some cases you can use "Piece of cake" instead an adverb phrase "At once".

Piece of cake noun - Any undertaking that is easy to do.

At once and piece of cake are semantically related. Sometimes you can use "At once" instead an adverb phrase "Piece of cake".