...I.E. can I put a layout I designed at work on my personal website advertised as work that I have created?
...and then wait for a cease and desist? Sure, why not?
A better solution would be to ask your employer for permission? ...and the best solution would be to get the permission in writing.
I've asked, and received permission from employers to display art I've created while in their employ. Those layouts are attributed to the respective employers. I also have finished samples of some jobs to include in my private portfolio --- even then, with overruns of these jobs, I've asked the employer for those to include in my portfolio.
Of course, there are some works that have been restricted or denied, but those were obvious why they couldn't be in anyone's portfolio. Most were because of how ownership rights were addressed with the customer to produce the jobs.
Provided the art has appropriate attributions, and is free from any contractual obligations with the end-user not to use it, there shouldn't be a problem asking and receiving permission to use it as self-promotion.
...but I'm not a lawyer.