cakes, pastry
糕 centers on baked goods made from grain flour: cakes and pastry. The idea is a soft, sweetened food prepared from rice or wheat.
糕 combines 米 (rice) with 羔, which likely contributes the sound and possibly the idea of something soft or steamed, as 羔 originally referred to a lamb. The exact historical development is uncertain.
The left side is 米 (rice), and the right side 羔 looks like a lamb with a tail. Imagine a soft rice cake shaped like a little lamb, fresh from the steamer.
For コウ, picture a cake so delicious it makes you say 'coo!' (like a dove): coo -> コウ, and the cake is a pastry made from rice.