1. Put the chocolate pieces in a large bowl and set a mesh strainer over the top.
2. Warm the milk, sugar and salt in a medium saucepan. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm mixture into the egg yolks, whisking constantly, then scrape the warmed egg yolks back into the saucepan.
3. Stir the mixture constantly over medium heat with a heatproof spatula, scraping the bottom as you stir, until the mixture thickens and coats the spatula. Pour the custard through the strainer over the milk chocolate, then stir until the chocolate is melted. Once the mixture is smooth, whisk in the cream, then the Guinness and vanilla. Stir until cool over an ice bath.
4. Chill the mixture thoroughly in the refrigerator (at least 8 hours or overnight), then churn in your ice cream maker according to the manufacturer's instructions.