Is it permissible to take dessert recipes from the Internet for the purpose of earning money, while being unsure of their copyright, and given that they are published in forums without mentioning a source?

There is no objection to downloading cookbooks and dessert recipes from the internet and using them to make and sell desserts, whether or not they are protected by copyright, because personal use is permissible. What is forbidden is trading in these photocopied books or re-publishing them on the internet in a way that harms their authors or publishers. Sheikh Ibn Uthaymin issued a fatwa permitting the copying of computer discs and the photocopying of books for personal use without trading in them. The ruling does not differ whether the owners of the company are Muslims or non-Muslims.