Make the Brownies: Preheat oven to 325 degrees F. Line an 8x8-inch square baking dish with aluminum foil and coat lightly with non-stick cooking spray.
In a large bowl, whisk together the flour, cocoa powder, granulated sugar, brown sugar and salt until blended. Using a wooden spoon, stir in the melted butter and mix until combined. (The batter may get hard like fudge at this point.) Add the eggs and vanilla extract and stir until blended and no clumps remain. Fold in ½ cup of the chopped pecans. Pour the batter into the prepared baking dish and bake for 35 - 37 minutes, or until a thin knife or skewer inserted into the center comes out with very moist crumbs attached.
Remove from the oven and sprinkle the marshmallows evenly over the top. Return the pan to the oven and bake until the marshmallows are slightly puffy, about 4 minutes. Remove from oven and place on a wire cooling rack. Allow the brownies to rest for 5 minutes, then toast the tops of marshmallows with a kitchen torch (this is optional).
Make the Chocolate Fudge Icing: In a small saucepan, melt the butter over medium-high heat, stirring frequently. Add the milk and cocoa powder and cook, whisking constantly, until the mixture has thickened, 1 to 2 minutes. Remove from the heat and add the powdered sugar, vanilla and salt, and beat with an electric mixer on medium speed until the frosting is smooth.
Immediately pour and spread the chocolate icing evenly over top (work quickly as the icing begins to harden as soon as it's made). Sprinkle the top with the remaining chopped pecans. Allow to cool completely and cut into squares. The brownies can be stored in an airtight container at room temperature for up to 5 days.