Paul Oliver sheds some light in his 'Screening the Blues':

The distinction between metaphor, euphemism and sexual terminology is therefore not easily determined. There is never any doubt that 'jelly' means sexual pleasure in Negro parlance and leads, by a complex of sexual associations to many extensions. 'Jelly bean' becomes a lover, 'jelly roll' coitus or, in some instances, ejaculation, and so 'jelly' becomes the semen, the rhythms of intercourse, the skill itself ... By extension, dough-rolling, biscuit-rolling, bread-baking and other baking terms have stemmed from the 'jelly' image to become in themselves widely understood terms of sexual significance. [Paul Oliver 'Screening the Blues' Da Capo 1968, pp 208-209].

--Stewie.