NEW YORK — In its latest push to establish itself as a purveyor of premium burgers, Wendy’s is testing a pretzel bun.

The Dublin, Ohio-based chain has started selling a bacon cheeseburger in select locations made with a slightly heartier, pretzel-like bun. A spokesman for Wendy’s declined to confirm the tests or provide any details, such as its price or where it’s being sold. But the ‘‘Pretzel Bacon Cheeseburger’’ was being advertised at a Wendy’s in Miami with others posting pictures and talking about it on Twitter.