Preheat the oven to 350℉ (175℃) and lightly grease a 9×13-inch baking dish. In a medium bowl, sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt, then set aside. In a large bowl, blend together the oil, eggs, vanilla, milk, and sour cream using an electric mixer until the mixture is smooth, taking care not to overmix. Reduce the mixer speed to low and slowly incorporate the boiling water, then beat at high speed for one minute to aerate. Combine the sifted dry ingredients with the wet mixture and mix until just fully incorporated.

Transfer the batter into the greased pan and bake for approximately 30 minutes or until a toothpick inserted into the center emerges clean. Once baked, remove the cake from the oven, evenly distribute the mini marshmallows on top, and return it to the oven for an additional 2 minutes until the marshmallows begin to soften. Allow the cake to cool completely.

To prepare the frosting, melt the butter in a small saucepan. Stir in the cocoa powder and heavy cream, cooking over medium heat for about 2 minutes while stirring continuously. Take the pan off the heat and blend in the vanilla and powdered sugar using an electric mixer until the mixture is smooth. Pour the warm frosting over the cooled cake and refrigerate for 25 to 30 minutes or until set.