In a medium suacepan, combine sugar, cocoa powder, salt, and milk. Over medium heat, bring mixture to a simmer while stirring contantly.
Separate four egg yolks into a small bowl and beat eggs lightly with a fork until well blended. Slowly add approximately ½ a cup of the sugar/milk mixture to the eggs while gently stirring (this is going to cook the egg yolks). Once the eggs are well incorporated in the small bowl add it to the remainder of the sugar/milk mixture on the stove.
Reduce your heat on the stove to low-medium heat and stir the mixture until it has thickened--you want this to have the same consistency as a thick gravy. Once the mixture has reached the desired consistency add in the semi-sweet chocolate. Continue to stir mixture until the chocolate has completely melted, then remove from heat.
Pour mixture into a glass bowl. Cover with plastic wrap and chill in fridge for 2 hours. It is important to use a glass bowl because it chills your mixture faster and more evenly--if your mixture is not really cold it will not work!
Remove chilled mixture from fridge and stir in chilled cream, vanilla, and chilled guiness until well incorporated. Pour into the ice cream maker.
Churn ice cream in the ice cream maker for approximately 20-40 minutes, or until the desired consistency. This step will depend greatly on what type of ice cream maker you have, please follow the instructions that came with your ice cream maker--mine turns off on its own when it is done.
Spoon into a freezer safe container and freeze for at least 1 hour before serving. I prefer freezing overnight before serving.