The term 'Cognitive disability' is a broad term that includes users who have difficulty with mental tasks. This can cover issues with solving problems, memory and attention, understanding language, and so on.
Of all the disabilities, users with cognitive disabilities are probably the most difficult to accommodate. It is such a new field, particularly in its relation to the Web, that methods of accommodating this user groups' needs are still being developed. In short, it is hard to find definitive evidence of what does and doesn't work.
However, use of good and clear design and site layout, intuitive information architecture, simple interfaces and language in your website are useful techniques which will help. Indeed, you can use these rules of thumb to not only help users with cognitive disabilities, but every one else as well!