Preheat oven to 350 F. Grease a 9x13-inch baking pan. Set aside.In a medium bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt. Set aside.In a large bowl, whisk together buttermilk and vegetabl oil.Whisk in eggs one at a time.Stir in dry intredients until just combined.Add hot water and vanilla.Whisk until just combined.Pour batter into prepared baking dish.Bake 30-40 minutes, or until wooden toothpick inserted in the center comes out clean.Cool to room temperature.To make the chocolate mousse, add mini marshmallows, chocolate, and milk to a large, heavy saucepan.Cook over low heat, stirring constantly, until melted and well-combined.Remove from heat, and cool completely.In a large bowl, beat whipping cream and vanilla together.Fold whipped cream into cooled chocolate mixture.Spread over cooled cake.Chill for at least 30 minutes, or until ready to serve.Refrigerate any leftovers.