01 - Whisk together whole milk, heavy cream, granulated sugar, unsweetened cocoa powder, cornstarch, and salt in a medium saucepan until the mixture reaches a smooth consistency.
02 - Set the saucepan over medium heat and cook, whisking continuously, until the mixture thickens and starts to bubble, approximately 8 to 10 minutes.
03 - Remove the saucepan from heat. Add pure vanilla extract and chopped semisweet chocolate, whisking until the chocolate is fully melted and the pudding is silky.
04 - Divide the pudding among individual glasses or serving cups, filling each about two-thirds full. Cover each with plastic wrap, pressing directly onto the pudding surface to prevent a skin from forming. Chill in the refrigerator for at least 1 hour.
05 - Once chilled, top each serving generously with crushed chocolate sandwich cookies to resemble soil.
06 - Decorate each glass with gummy worms, pressing them slightly into the cookie layer so they appear to be emerging. Serve cold.