Green’s Dictionary of Slang

poundcake n.

1. (US) an attractive woman.

[US]Wentworth & Flexner DAS.

2. (US gay) the buttocks.

implied in eat poundcake under eat v.
[US]E. Sanders [title] Bugger! An Anthology of Anal Erotic, Pound Cake Cornhole, Arse-Freak and Dreck Poems.
[US]B. Rodgers Queens’ Vernacular.
[US]R.O. Scott Gay Sl. Dict. [Internet].

3. (US gay) coprophilia.

[US]R.A. Wilson Playboy’s Book of Forbidden Words 47: Those who seek the more thorough masochism of being defecated upon ask for pound cake.

4. (US) pornographic writing or magazines.

[US]J. Stahl Permanent Midnight 37: The heady world of magazine porn, or ‘poundcake,’ as it’s known among the pros.

In phrases

make poundcake (v.)

(US gay) to have anal intercourse.

[US]R.O. Scott Gay Sl. Dict. [Internet] anal intercourse: [...] Syn: making poundcake.