In printing, an "em dash" is the term used to describe the longish dash that sets off a word or phrase within a sentence—like this!
Since typewriters didn't have an em-dash key, two hyphens became the standard substitute--like this.
Either way is considered correct, nowadays. But if you like the look of the em dash, you can make it by pressing the shift, option, and hyphen keys at the same time. Like—this—! Ooh, how very dashing!
Emily Dickinson would approve.