Preheat oven to 375° F. Lightly grease bottom of a 13×9-inch baking pan with butter or shortening. Whisk together in large bowl the cocoa powder, flour, baking soda and salt. Then set aside.
Melt butter in a medium saucepan over low heat. Add dark brown sugar, stirring until completely blended with butter. Stir flour mixture into the butter mixture. When mixed, remove from heat and set aside. The mixture at this point will be very dry.
In medium bowl, whisk eggs and vanilla. Incorporate egg mixture into brownie batter in the pan. Fold in 2 cups pretzels and 1 cup milk chocolate chips. Pour into the prepared baking pan.
In medium bowl, stir together marshmallows, peanuts, 1/2 cup pretzels and 1/2 cup milk chocolate chips. Sprinkle evenly over the top of brownie batter.
Bake for 25 minutes, or until set. Let brownies cool completely, about 1 hour. Cut into 6 rows by 4 rows to make 24 brownies.