on master:
git checkout -b newbranch or as suggested below, simply do a git branch newbranch to create the new branch without switching to it.
This will create a branch that is at your current commit at master. Once done:
git checkout master
followed by:
git reset --hard <commit_hash>
Where <commit_hash> should be replaced by the commit ID you want master rolled back to.
Now you can switch to your new branch and push it out to the remote.
git checkout newbranch
git push origin newbranch