My basic brunch.io config (for things like gohugo.io) I keep my stuff in ./assets/ so that's why public is ../public

brunch-config.js

exports.files = {
  javascripts: {
    joinTo: 'js/app.js'
  },
  stylesheets: {
    joinTo: 'css/app.css'
  }
}

exports.conventions = {
  assets: /^(static)/
}

exports.paths = {
  watched: ['static', 'css', 'js', 'vendor'],
  public: '../public'
}

exports.plugins = {
  babel: {
    presets: ['env'],
    ignore: ['vendor']
  },
  postcss: {
    processors: [
      require('postcss-import')(),
      require('postcss-css-variables')(),
      require('postcss-custom-media')(),
      require('autoprefixer')(['last 8 versions']),
      require('csswring')()
    ]
  }
}

exports.modules = {
  autoRequire: {
    'js/app.js': ['js/index']
  }
}

exports.npm = {
  enabled: true
}

package.json

{
  "name": "some-website",
  "version": "0.0.1",
  "description": "A website",
  "private": true,
  "scripts": {
    "dev": "brunch build",
    "start": "brunch watch --server",
    "build": "brunch build --production"
  },
  "repository": {
    "type": "git",
    "url": "https://the-git-hub/the-repo.git"
  },
  "author": "Me",
  "license": "MIT",
  "dependencies": {
  },
  "devDependencies": {
    "auto-reload-brunch": "^2",
    "autoprefixer": "^7.1.4",
    "babel-brunch": "~6.0",
    "babel-preset-env": "^1.6.0",
    "brunch": "^2",
    "clean-css-brunch": "^2",
    "csswring": "^6.0.1",
    "postcss-brunch": "^2.1.0",
    "postcss-css-variables": "^0.8.0",
    "postcss-cssnext": "^3.0.2",
    "postcss-custom-media": "^6.0.0",
    "postcss-import": "^10.0.0",
    "tachyons": "^4.8.0",
    "uglify-js-brunch": "^2"
  }
}