Preheat oven to 350 degrees F. Lightly grease a 9x13'' baking pan.

In a large mixing bowl combine the flour, sugars, cocoa powder, baking powder, baking soda and salt.

Add oil, eggs, vanilla, milk and sour cream and mix until combined. Reduce speed and carefully pour in the boiling water. Beat on high speed for 1 minute to add air to the batter.

Pour batter into prepared pan. Bake for 38-45 minutes, or until a toothpick inserted in the center comes out clean.

Remove cake from oven and sprinkle marshmallows all over the top of the hot cake, completely covering it.

Return to oven (at 350 degrees F) for 3-5 minutes or until the marshmallows are just slightly puffed and melted.

Allow cake to cool completely before drizzling frosting on top.

Melt butter in a small saucepan over medium-high heat.

Add milk and cocoa powder and whisk well. Cook for about 2-3 more minutes, stirring constantly.

Remove from heat and stir in vanilla and powdered sugar. Use an electric mixer to make the frosting smooth.

Drizzle warm frosting immediately over cake. Refrigerate cake for about 30 minutes or until frosting has set. Store at room temperature.

Next time try Mississippi Mud Brownies!