If you are looking to make American-style cakes and desserts and happen to be in the center of The Hague, I would actually recommend visiting the Asian store underneath C&A (to the left of the C&A entrance you will see stairs leading downwards – see also this blog post with a photo).
Here is just a small sampling of the various Duncan Hines recipes they offer:
In total there are probably about 15-20 different types on offer at any point. German chocolate cake, red velvet cake, blue velvet cake, coconut surprise… Unfortunately the selection isn’t available in the webshop on their website linked above. Of course if you are desperate for American food there is also usfoodz.nl though it is definitely import prices.
As there is a large Asian population in the area (it is not far from Chinatown) the store is almost always crowded. But it is a staple if you’re looking for Asian food. There’s also a smaller store by the same name in the Markthof, across from the Hema entrance and close by Simonis, a fresh fish store, and “Flowers for you” (a flower shop, of course).