In a large bowl, whisk together the egg yolks, 1/4 cup sugar and the salt. Whisk in the cocoa powder until smooth. In a medium saucepan, combine the milk, cream and remaining 1/4 cup sugar over medium heat and cook, stirring, until steaming hot but not simmering, about 6 minutes. Add to the egg mixture in a slow stream, whisking constantly, then pour into the saucepan. Cook over medium low heat, stirring constantly, until the custard thickens and registers 175 degrees on an instant-read thermometer, about 8 minutes.
Strain the custard into a medium bowl set in an ice water bath. Let cool, then cover and refrigerate for at least 2 hours.
Using an ice cream machine, process the custard according to the manufacturers instructions. Fold in the chocolate chips and pretzels, then transfer to an airtight container and freeze until firm, about 4 hours.