"Help yourself" is fine. You could also say "Have some." "Treat yourself" is okay, but it implies a context that the receiver is reluctant to take a piece., so you are encouraging him/her. Also, we say "bake" for things that are cooked in the oven like cakes or pies: "I've *baked* a cake."

If they're good friends, I'd usually tell them to "have at it" and they'd know what I mean. You could also say "try a piece".

If you said help yourself, that would only be after placing it on the table. If you want to offer before you have taken the cake out of the kitchen you would say, "Would you like some cake" or "Would anyone like some cake"

"Would you like to try this cake I made?" "Please, try some cake." "I baked this cake, will you please try a piece?" Those are just a few ways you can get them to try your cake.