Not only is the joke itself funny(well, it was when I was a kid anyway), the story behind me trying to tell it is funny as well.

In 1980 my husband (Beirut_Veteran) and I got married and were on our way to California to start our life. On our long journey from Florida to California, we told jokes to pass the time. After listening to thousands of jokes that my husband had aquired through sailor buddies I finally tried my hand at it.
Problem was, I forgot the stupid punch line! It wasn't until 1990 that I remembered the whole joke. Here it is.

A little boy runs up to an ice cream truck and asks the ice cream man if he can have some chocolate ice cream. The man looks kindly at the little boy and says "Sorry sonny, but I'm out of chocolate. How about a nice vanilla or strawberry cone? " The boy looks at the menu on the side of the truck and asks again. "Can I have a chocolate and strawberry ice cream cone?" The man looks at the boy in confusion. "Sonny, I just told you I don't have any chocolate ice cream today. I have vanilla and strawberry." The boy glances at the menu again. "Okay, I'll have a chocolate and vanilla swirl then." The man gets irritated and steps out of the truck. "Sonny, do me a favor. Spell the "van" in vanilla. The boy looks at the man as if he had lost his mind as he spells it out. "V-A-N". The man nods his head. Now spell the "straw" in strawberry. The boy spells it out easily. "S-T-R-A-W". The man nods. Now spell the "f**k" in chocolate. The boy's eyes get huge as he shouts "There is no F**K in chocolate!" The man smiles and gets back into his truck. "That's what I've been trying to tell you all along sonny!" :p