Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper leaving an overhang on the sides for easy removal.
Step 2: In a large saucepan or microwave-safe bowl, melt 1 cup of butter. Remove from heat and stir in the granulated sugar until well combined.
Step 3: Beat in the 4 large eggs one at a time, then stir in 1 teaspoon of vanilla extract.
Step 4: Gradually add the 1/3 cup cocoa powder, 1 1/2 cups all-purpose flour, and 1/4 teaspoon salt to the wet ingredients, mixing until just combined. Do not overmix.
Step 5: If using, fold in the 1 cup of chopped pecans. Pour the batter into the prepared baking pan and spread evenly.
Step 6: Bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs. Do not overbake for fudgier brownies.
Step 7: Remove the brownies from the oven and immediately sprinkle the 4 cups of mini marshmallows evenly over the hot surface. Return to the oven for 3-5 minutes, or until marshmallows are puffed and lightly golden.
Step 8: While the brownies are cooling slightly, prepare the frosting: In a medium saucepan, melt 1/2 cup butter over medium heat. Stir in 1/3 cup cocoa powder and 1/3 cup milk, whisking until smooth. Bring to a gentle boil, then remove from heat.
Step 9: Stir in 1 teaspoon vanilla extract. Gradually add the 3 1/2 cups powdered sugar, whisking until the frosting is smooth and lump-free. If too thick, add a tiny bit more milk; if too thin, add a bit more powdered sugar.
Step 10: Once the brownies have cooled completely (this is crucial for clean cuts and a stable frosting), pour the warm chocolate frosting over the marshmallow layer and spread evenly.
Step 11: Let the frosting set at room temperature for at least 1-2 hours before slicing and serving. For cleaner cuts, you can refrigerate for 30 minutes to speed up the setting process.