Why not 巧克力 的 蛋糕？
Following the hover-over hints I put 片 (lit. 'slice') instead of 块. Ho, hum, wrong again, or is it?
I lived in a Chinese speaking country for a few years, and they used 片
请给我一片巧克力蛋糕; I'm hoping they accept this, although it might be more colloquial