Preheat your oven to 350°F (175°C) and lightly grease a 9×13-inch baking pan. This will ensure your cake doesn’t stick and comes out perfectly.
In a large bowl, whisk together the flour, granulated sugar, cocoa powder, salt, baking powder, and baking soda. Mixing these dry ingredients first helps to evenly distribute the leavening agents, ensuring a well-risen cake.
Add the milk, vegetable oil, and eggs to the dry mixture. Stir until everything is well combined. The batter may look a bit thick, but don’t worry!
Now, stir in the boiling water (or coffee if you prefer). Be cautious, as the batter will be quite thin. This is what makes the cake so moist!
Pour the batter into the prepared pan and spread it evenly. A spatula works wonders here, ensuring every corner is filled with that chocolaty goodness.
Bake for 25 to 30 minutes. Check for doneness by inserting a toothpick into the center. It should come out with a few moist crumbs, but not wet batter.
Once baked, remove the cake from the oven and immediately sprinkle the mini marshmallows evenly over the top. Watching them melt is a sight to behold!
Return the cake to the oven for an additional 2 to 3 minutes, just until the marshmallows puff up slightly and begin to melt. This creates a delightful, gooey layer.
Remove the cake once more and allow it to cool slightly while you prepare the frosting. The waiting is the hardest part!
In a medium saucepan over medium heat, melt the salted butter. Once melted, whisk in the powdered sugar, cocoa powder, and milk until no lumps remain. Make sure to stir continuously to prevent burning.
Finally, add in the chopped pecans and stir for one minute before removing from heat. You don’t need to boil the mixture, just heat it through nicely.
Immediately pour the chocolate frosting over the marshmallow-topped cake, ensuring to cover the marshmallows completely. This will create a rich chocolate layer that everyone will love!
If desired, sprinkle additional chopped pecans on top for garnish. Allow the cake to cool completely before slicing and serving. Enjoy every bite of your Mississippi Mud Cake!