Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan and line it with parchment paper, leaving an overhang on the sides to easily lift the brownies out later.

In a large mixing bowl, whisk together the melted butter and granulated sugar until well combined.

Add the room temperature eggs one at a time, whisking well after each addition. Stir in the vanilla extract.

In a separate medium bowl, whisk together the all-purpose flour, sifted cocoa powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing with a rubber spatula until just combined. Be careful not to overmix.

Pour the batter into the prepared pan and spread it into an even layer. Bake for 28-32 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs attached, but not wet batter.

Remove the brownies from the oven and immediately sprinkle the mini marshmallows evenly over the top.

Return the pan to the oven and bake for another 2-3 minutes, just until the marshmallows are soft and puffy. Watch carefully to prevent them from browning too much.

While the marshmallows are melting, prepare the frosting. In a small saucepan over medium heat, melt the 1/2 cup of butter.

Remove the saucepan from the heat and whisk in the sifted cocoa powder until smooth. Whisk in the milk and vanilla extract.

Gradually whisk in the sifted powdered sugar until the frosting is smooth and spreadable. If it's too thick, add another teaspoon of milk. Stir in the chopped pecans.

Immediately pour the warm frosting over the puffy marshmallow layer, spreading it gently with an offset spatula to cover the entire surface.

Let the brownies cool completely in the pan on a wire rack for at least 2-3 hours, or until the frosting is set. For cleaner cuts, you can refrigerate them for 30 minutes before slicing.

Use the parchment paper overhang to lift the brownies out of the pan. Cut into 16 squares and serve.