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));
});

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.