To say “Goodbye” among friends or “I love you” to someone who you are starting to date without actually saying those three words of commitment or just not to be cheese.
Note: This needs to be used between two people. One saying pie and the other cake.
To say I love you to someone and they say it back without both of you wanting true commitment, or niether of you are ready to say "I love you" yet.
(Girlfriend and boyfriend are finishing up talking on the phone)
Anna: Babe, I'll talk to you tomorrow.
Dave: Ok.
Anna: Piece of cake, piece of pie.
Dave: Piece of cake, piece of pie.
(both hang up the phone)
This is when you are flirting with the opposite sex and you want to get their attention to have a bang, you say "hey, fancy a piece of my pie? Its moist and delicious".