Preheat oven to 350°F (175°C). Grease two 8-inch round pans.
In a large bowl, whisk flour, sugar, cocoa, baking soda, baking powder, and salt.
Add eggs, buttermilk, oil, and vanilla. Mix until smooth.
Slowly stir in hot coffee or water. Batter will be thin.
Divide batter evenly between pans and bake 30–35 minutes. Cool completely.
Beat butter until creamy. Add powdered sugar, cocoa, milk, and vanilla. Beat until fluffy.
Stack cake layers with frosting between. Frost entire cake smoothly.
Press crushed cookies onto the top for dirt.
Pipe a blue frosting river across the surface.
Arrange wafer cookies in a small pile for the campfire.
Add marshmallows and candy flames in the center.
Pipe green trees around the edges and add candy rocks if using.
Chill briefly, slice, and serve.