Is This Correct?

Not really.

I would suggest, "After we eat the cake it will be time to leave."

When you say, "FOLLOWING WE EAT THE CAKE IT WILL BE TIME TO LEAVE," you are using 'following' as a preposistion.

The example I found of using 'following' as a prep. is, "Following dinner, brandy was served in the study."

Hope that helps.

Ciao!