jarv.is/plugins/netlify-plugin-amp-optimizer/index.js

const ampOptimizer = require("@ampproject/toolbox-optimizer").create({
  verbose: true,
});
const fs = require("fs");
const glob = require("glob");

module.exports = {
  onPostBuild: async ({ constants, utils }) => {
    // Hugo renders my AMP pages as amp.html right next to each page's index.html
    const pattern = constants.PUBLISH_DIR + "/**/amp.html";

    const files = await new Promise((resolve, reject) => {
      glob(pattern, { nodir: true }, (err, files) => {
        err ? reject(err) : resolve(files);
      });
    });

    await Promise.all(
      files.map(async (file) => {
        const html = await fs.promises.readFile(file, "utf-8");
        const optimizedHtml = await ampOptimizer.transformHtml(html);
        await fs.promises.writeFile(file, optimizedHtml);
      })
    );

    utils.status.show({
      title: `${files.length} AMP pages optimized`,
      summary: "Great success! ⚡",
    });
  },
};