• 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.

For the chocolate ganache frosting:

• In a small saucepan, bring cream to a simmer.
• Stir in finely chopped semisweet chocolate. Continue stirring until chocolate mixture is smooth, glossy, frosting forms.
• Pour the warm icing over the top of the cake. Let ganache set in refrigerator before slicing and serving.