Place 2 cups milk in a medium saucepan with sugar and cocoa powder over medium heat until steaming and warm. Whisk the mixture gently while it's heating up.

In a medium bowl, whisk together remaining ½ cup milk, cornstarch, salt, egg yolks, and mint extract. Drizzle about half of the steaming chocolate milk into the egg mixture, whisking constantly. This will temper the eggs so that they warm up slowly and don't become scrambled eggs when they're added to the pan on the stove. Add the egg mixture to the saucepan and heat over medium heat, whisking constantly. Be sure to scrape the corners of the pan so that the pudding doesn't overcook around the sides.

When pudding comes to a rolling boil, continue to whisk and boil for another 2 to 3 minutes as pudding thickens. Remove pudding from heat and spoon into ramekins or jars. Cover each with plastic wrap so it's touching the top of the pudding to avoid pudding skin. Refrigerate the pudding for at least 4 hours before serving.

When ready to serve, crumble the chocolate wafer cookies or smash them with a mallet and cover the top of each ramekin with cookie crumbles. Garnish each ramekin with a mint sprig and serve.