When you make a cake and rub the left over frosting on your girl's tits, then titty fuck her. Then she sucks the frosting off your dick like she is blowing out the candle.
On my birthday, after my party. My girl took me to the bedroom and we did the birthday cake.
Having sex with a girl from behind, and finishing on her ass. After you've finished you use a rubber spatula to evenly distribute the cum on her butt cheeks, as if it were icing on a cake. When this is complete you may stick a candle in her butt hole and light it. Finish off by putting on a birthday party hat and you have successfully completed The Birthday Cake.
"Dude, I gave Steph the birthday cake last night!"
"Oh that's nice of you dude, was it red velvet or...?"
"No dude, I came on her butt and smeared it with a spatula."