To me, ellipses are for pauses to think, or for trailing off, the sentence either left incomplete or finished only inside the speaker's head. (There are apparently quite a few websites that disagree with me, which is not news.)
As far as I know, there's not an agreed-upon way to punctuate a stammer (although there is for a stutter, oddly enough). But there's a general agreement on the difference between a stutter and a stammer, and maybe it informs your punctuation choice.
A stutter is dialogue that gets hung up on the initial sound of a word. It can be a s-s-single letter or an initial sound, like th-th-th-this. (And that's how you punctuate it.) A stammer is dialogue that hangs up on an entire word, or according to some, a complete syllable.
So I guess what I want to know is whether she's really stammering, if she's unable to collect her thoughts in time to say the words, or if she's stopping herself by censoring as she speaks. (It's all so complex, eh?)
Stutter:
Sh-sh-she went out the b-b-b-b-back!
Stammer:
She went, went, went out the back!
Groping for words:
She... She went... out the back!
Self-interrupting inside one's head:
She went-- out the back!
Maryn, hoping this makes sense