To make the fudge swirl:
Heat the cream, butter, and sugars in a small saucepan over low heat. Bring the mixture to a boil and cook until the sugar dissolves, about 2 to 3 minutes. Remove from heat and stir in the peanut butter and chocolate chips. Let stand for 4 to 5 minutes before stirring completely smooth. Stir in the salt and vanilla. Let the mixture cool to room temperature before using. Fudge swirl can be stored in an airtight container in the fridge for up to 2 days. Bring to room temperature before using.
For the ice cream:
In a medium bowl, whisk the cocoa, sugars, and salt. Add in the milk and using a hand mixer on low speed, beat to combine so everything is fully dissolved. Stir in the heavy cream, and vanilla. If mixture is very cold, move onto churning in the ice cream maker. If not, refrigerate, covered, until well chilled, 2 hours or overnight.
Pour the mixture into an ice cream maker and freeze according to the manufacturer’s directions. In the last five minutes of churning, add in the brownie chunks. Transfer the ice cream to an airtight container in batches, spooning the fudge swirl in between layers of ice cream in a swirled pattern. Press plastic wrap against the surface of the ice cream and freeze until it is firm, about 3 hours, before serving. If the ice cream becomes too frozen to easily scoop, let it stand at room temperature for 5 to 10 minutes before scooping.