In a medium skillet, heat 1/2 of the beer over medium heat and simmer until reduced by half (about 10 minutes).
In a medium saucepan, warm 2 cups of the cream and whisk in the cocoa powder until dissolved. Bring to boil, then reduce the heat and simmer at a low boil for 30 seconds, whisking constantly. Remove from heat and add the chocolate, stirring until smooth. Stir in the remaining cream, porter, and vanilla extract. Pour the mixture into a large bowl.
Warm the milk, sugar, and salt in the saucepan. In a separate medium bowl, whisk together the egg yolks. Slowly pour the warm milk into the egg yolks while whisking constantly so that you do not cook the egg yolks.
Stir the mixture constantly over medium heat, scraping the bottom as you stir, until the mixture thickens into custard and coats the back of the spoon. The custard can quickly begin to curdle if it reaches temperatures over 170 degrees F, so it is best to check the temperature with a thermometer. Pour the custard into the chocolate and beer mixture until smooth, then stir in one quarter of the peanut butter until completely incorporated.
Chill the mixture thoroughly in the refrigerator (at least 4 hours or preferably overnight).
Freeze in your ice cream maker according to the manufacturer’s instructions. Swirl in the remaining peanut butter and freeze overnight or until solid.