First, make sure your ice cream maker's bowl has been chilled in the freezer overnight! I always just keep my bowl in the freezer so I don't forget! Prepare an ice bath in a large mixing bowl and set aside.

Place the chopped dark chocolate in a heatproof bowl. Bring a large saucepan of water to a boil (or any pan that the bowl can rest on without touching the water). Once boiling, reduce heat to a low simmer and rest the bowl on the pan (if you have a double boiler, that works great too!) Stir the chocolate until it is melted and smooth. Set aside.

In a separate saucepan, bring the milk, cream, sugar, brown sugar, sweetened condensed milk, vanilla, cocoa powder, and salt to a simmer over medium-high heat. Reduce heat and simmer for 1-2 minutes, whisking the mixture constantly until smooth, being very careful not to let the mixture boil. 

Pour a ladleful of the milk mixture into the chocolate and whisk quickly until smooth. Add another ladle of the milk mixture and working quickly, whisk again until smooth. Repeat until you've added half of the milk mixture, and then add the remaining and whisk to combine until smooth and incorporated.

Strain ice cream mixture through a fine mesh strainer into a bowl (this will get rid of any clumps and make your ice cream super silky!) Place the bowl in your ice bath (but make sure that no water overflows into the ice cream mixture!) and chill for 30 minutes or so, or until mixture has cooled completely.

Transfer the ice cream mixture into the bowl of your ice cream maker. Churn for about 20-25 minutes, or until your ice cream has reached the consistency of a thick soft serve. During the last couple minutes, add half of the peanut butter gradually in spoonfuls, as well as the pretzel pieces. During the last few seconds of churning, add the remaining few spoonfuls of peanut butter, so that it barely gets mixed in and creates "ribbons" of peanut butter in the ice cream.

Transfer the ice cream to a loaf pan lined with parchment, or a freezer safe container. Swirl additional peanut butter into the top of the ice cream, and sprinkle with extra crushed pretzels, if desired. Cover with plastic wrap or wax papered press against the surface to prevent ice crystals from forming. Freeze until solid, at least 4 hours. Enjoy!