Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Place shortening and chocolate squares in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Remove from heat.
Stir cocoa mix into the melted chocolate; mix in sugar and eggs. Add flour slowly and mix thoroughly. Mix in marshmallows. Pour batter evenly into the prepared pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Let cool.