gulpfile.js

gulp.task('sass', function() {
  return gulp.src('assets/sass/**/*.scss')
    .pipe(plumber(plumberOptions))
    .pipe(sass(sassOptions))
    .pipe(autoprefixer(autoprefixerOptions))
    .pipe(gulp.dest('assets/css'))
    .pipe(filter(sassFilterOptions))
    .pipe(reload(sassReloadOptions));
});

gulp.task('sass-jekyll', function() {
  return gulp.src('assets/sass/**/*.scss')
    .pipe(plumber(plumberOptions))
    .pipe(sass(sassOptions))
    .pipe(autoprefixer(autoprefixerOptions))
    .pipe(gulp.dest('_site/assets/css'))
    .pipe(filter(sassFilterOptions))
    .pipe(reload(sassReloadOptions));
});

@jonsuh, correct me if I'm wrong, but I'm pretty sure you can pipe a second destination in there.

gulp.task('sass', function() {
  return gulp.src('assets/sass/**/*.scss')
    .pipe(plumber(plumberOptions))
    .pipe(sass(sassOptions))
    .pipe(autoprefixer(autoprefixerOptions))
    .pipe(gulp.dest('assets/css'))
    .pipe(gulp.dest('_site/assets/css'))
    .pipe(filter(sassFilterOptions))
    .pipe(reload(sassReloadOptions));
});

That being said, I've used the minimist & gulp-if approach to switch the build destination with a flag before, which does work well for creating a production build before pushing to the production server.

Todd, your approach does work, but in my specific scenario I'm looking to avoid it. The reason is because it triggers Jekyll’s auto-regeneration, and relying on Jekyll to auto-regenerate the changed CSS is slow, which is why I have a separate task to build the CSS directly into Jekyll’s build folder _site.