Heat oven to 350 F. Grease and line an 8 x 8 inch square cake pan with parchment paper.
In a large mixing bowl, sift together dry ingredients (flour, sugar, cocoa powder, baking powder, baking soda, salt).
Add wet ingredients (oil, eggs, milk, vanilla extract, and sour cream) to the flour mixture and use an electric mixer on medium speed to combine.
Pour hot water in and mix on low speed to combine.
Pour batter into the prepared pan and bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Remove hot cake from the oven and sprinkle marshmallows over the top, covering it completely.
Return to oven for 2-3 minutes until mini marshmallows are slighlty puffed. Cool cake before drizzling the warm frosting over the top of the fluffy marshmallows.