In a medium saucepan off heat, whisk together egg yolks and sugar until light in color and slightly thickened, 3 to 5 minutes. Whisk in cocoa powder until no lumps remain. Slowly add whole milk and heavy cream, whisking constantly. Finely chop 10 Thin Mints (crumbs should be no larger than a pea) and stir into dairy.

Put saucepan on medium-low heat and cook, whisking frequently, until custard reaches 180°F on an instant read thermometer (happens right before it boils; it will thicken suddenly.) Remove custard from heat and stir in chocolate chips, then salt to taste.

When chocolate is fully melted and incorporated, transfer into an airtight container and chill overnight in the refrigerator.

The next day, churn in an ice cream maker according to manufacturer's instructions. Roughly chop the 18 remaining Thin Mints; chunks should range from pea to nickle-sized. In the last minute of churn, add chopped Thin Mints. Return ice cream to container and chill in freezer for at least two hours before serving.