I think the first thing that jumps out at me is that header. I'm not sure the dimensions and resolution of your screen, so maybe it's just an illusion, but if that were my screen it would mean that header is like 200+px tall. Header size can vary a bit, depending on several factors, but I'd keep it under 100px. GDSE (this site) is 50px tall, for some reference. I realize their logo requires a bit taller of a header, which brings me to my next point.
That logo needs a lot of work. I'm not sure if that falls into the scope of your contract with them, but that logo does not scale well and is too busy to easily read/recognize.
The cake image on the left looks too simple and too small. I'd make it bigger. You could even try it the other way around, showing a cake with a slice missing (example below). You could place the cake on a tray and add some other decorative/table items around it too.
If you really wanted to get crazy, you could use photographs of cake (assuming you have the resources to do so). If you can't get pics of their different variants, maybe you can use a photo of a vanilla cake and colorize it, based on the customer inputs.
It's hard to give too much other input, because we can only see such a small section of the design, but those are the things I immediately noticed