Both are okay. I laid out some desserts implies there were different kinds of desserts laid out whereas I laid some dessert implies it was the same dessert. Compare 'I laid out some sugar' vs 'I laid out some sugars'. Latter implies more than one type was laid out.

You could say either. You can think of desserts as countable, or uncountable.... dessert.

I agree with Robert. If I had lots of different types of desert I'd probably say DESSERTS. If I had a lots of desserts... but the same type, I'd probably say DESSERT.