Fill a large bowl with ice water and set aside. Begin by making chocolate liquor; combine cocoa powder, water, and 1/3 cup (67g) sugar in a small saucepan. Place saucepan over medium heat and bring to a low boil, whisking constantly. As soon as you see bubbles, remove from heat and add chopped chocolate. Let sit for 2 minutes and then stir chocolate liquor until smooth. Pour into a medium bowl and set aside.

In a small bowl, combine 2 Tablespoons milk with cornstarch, whisk and set aside.  Combine remaining milk, ½ cup (118ml) cream, salt, and 1/3 cup (67g) sugar in a medium saucepan and place over medium heat. Bring milk mixture to a low boil.  Cook until the sugar dissolves, 3 minutes.  

Remove milk mixture from heat and gradually whisk in the cornstarch mixture.  Return to a boil and cook over moderately high heat until the mixture is slightly thickened, about 1 minute.  Pour base into the chocolate liquor and whisk to combine. Stir in remaining 1 cup (237ml) cream. Set bowl in the ice water bath to cool, 20 minutes, whisking occasionally. Once cool, add vanilla. Refrigerate ice cream base until chilled, at least 4 hours or overnight.

Pour base into an ice cream machine and process according to the manufacturer's instructions. When churning is complete, gently fold in chocolate-covered pretzel pieces. Transfer to a freezer-safe container and freeze until firm, at least 4 hours.