In a medium saucepan, heat and stir chocolate pieces, condensed milk, butter, vanilla, and salt over medium heat until melted and smooth. Remove from heat; stir in marshmallows just until combined. Spread mixture evenly in the prepared pan. Using a table knife or thin metal spatula, swirl marshmallows through fudge in pan until marshmallows are mostly melted. Cover and chill about 2 hours or until firm.
Using a small scoop, scoop fudge and shape into rocks (fudge will be sticky). Roll some in cocoa powder mixture and some in powdered sugar. Store in an airtight container for up to 3 days. Makes about 80 pieces (2 1/4 lbs.).