Koo Kuih are turtle/tortoise shaped Chinese/Nyonya confectionary. They come in green, red, yellow, pink or even black colour skins. Those in red are usually given out to friends and relatives during a baby's full moon (1 month old) along with hard boiled eggs which are dyed red.
There are a few recipes online but I am putting a link to this recipe. See some pictures here.
The filling can be of green beans (both sweet or savoury) paste, coconut or peanuts/sugar mixture. I believe they are even available in lotus seed paste or red bean paste.
Anyway, my all-time favourite is the pandan (green version) filled with green/mung beans paste ones from Lulu Kuih Nyonya. They have stalls in most major shopping malls. They are very generous with the filling and the skin soft and thin.
I also like their siew paus. Of course, I like the Seremban siew paus better but the ones from Lulu Kuih Nyonya are not bad when there is a crave. I find the pastry from Lulu to be thicker and firmer which is OK if one prefers pastry to the filling.