Preheat oven to 350°F. Grease, or coat with Wilton cake release, a 9x13-inch baking pan.
In a medium bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
In a larger bowl or the bowl of an electric mixer, add buttermilk and vegetable oil. Mix. Stir in eggs one at a time.
Add in dry ingredients and mix until just combined. Add hot water and vanilla. Stir until just combined.
Pour batter into prepared baking dish. Bake 30-40 minutes, or until a wooden toothpick inserted in the center comes out clean.Cool to room temperature.
CHOCOLATE MOUSSE
Add mini marshmallows, chocolate, and milk to a large, heavy-bottomed saucepan. Cook, stirring constantly over low heat until melted and ingredients are well-combined.
Remove from heat, and cool to warm. (If it cools completely or is cold it will not mix well into the whipped cream.)
In a large bowl, beat whipping cream and vanilla together until soft peaks form. (Soft peaks barely hold their shape. The peaks flop over immediately when the beaters are lifted.)
Fold cooled chocolate into whipped cream. Spread over cooled cake. Chill for at least 30 minutes before serving. Refrigerate any leftovers.