Preheat your oven to 375°F (190°C) and grease two 9-inch round cake pans.

In a large bowl, mix the flour, sugar, cocoa powder, baking soda, and salt until well combined.

Add the eggs, milk, vegetable oil, and vanilla extract, mixing until smooth. Gradually pour in the hot water and stir gently to combine.

Divide the batter evenly between the prepared pans and bake for 30-35 minutes or until a toothpick comes out clean. Let them cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

In a small bowl, combine the graham cracker crumbs and sugar. Pour in the melted butter and mix until crumbly.

Spread the mixture on a baking sheet and bake at 350°F (175°C) for 5-7 minutes or until golden brown. Let cool.

In a large mixing bowl, beat the room temperature butter until creamy. Gradually add the marshmallow fluff and continue to beat until smooth.

Slowly add the powdered sugar, mixing until well combined. Stir in the mini marshmallows.

In a small saucepan, heat the heavy whipping cream over medium heat until steaming but not boiling.

Remove from heat and add the chocolate chips. Let sit for 2-3 minutes, then stir until smooth.

In another bowl, beat the room temperature butter until creamy. Gradually add the powdered sugar and mix until smooth.

Stir in the vanilla extract and milk or cream until you reach your desired consistency.