Originally from Family Guy, from the episode in which Mort Goldman said,"The kids were very mean to me in high-school. They used to call me tastycakes and shove pinecones in my ass."(As seen in Little Nicky)
Tastycakes is an adjective. It describes someone's ass. As if, it were tasty... As tasty as cake.
Tastykake is the brandname for a line of snack foods manufactured by the Tasty Baking Company of Philadelphia, Pennsylvania. Established in 1914 by Philip J. Baur and Herbert T. Morris and originally selling its product only in the Philadelphiametropolitan area, the company now distributes its products in several states on the East Coast.