Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a saucepan over medium heat, melt the butter. Stir in cocoa powder until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, beat the sugar, eggs, and vanilla until well combined.
Gradually add the cocoa mixture to the sugar mixture, stirring until smooth.
Stir in the flour and salt until just combined.
Pour the batter into the prepared baking dish and spread it evenly.
Sprinkle mini marshmallows evenly over the batter.
In a small saucepan, heat the milk until hot but not boiling. Remove from heat and stir in the chocolate chips until melted and smooth.
Drizzle the chocolate mixture over the marshmallows.
Sprinkle chopped nuts over the top if desired.
Bake in the preheated oven for 25-30 minutes, or until the edges are set but the center is still slightly gooey.
Allow the cake to cool before serving. Enjoy your delicious Mississippi Mud Cake!