Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper.

In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt until well combined.

Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat with an electric mixer on medium speed for 2 minutes, scraping down the sides of the bowl as needed.

Carefully stir in the hot water (or coffee) until the batter is smooth and thin. Be careful, as the hot liquid will make the batter very runny.

Pour the batter evenly into the prepared baking pan. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan on a wire rack for 15-20 minutes, then invert it onto the wire rack to cool completely. This is crucial before adding the filling.

In a large mixing bowl, cream together the softened butter until light and fluffy using an electric mixer.

Gradually add the sifted powdered sugar, mixing on low speed until combined, then increase to medium and beat until smooth.

Add the marshmallow fluff, heavy cream, and vanilla extract. Beat on medium-high speed for 3-5 minutes, until the mixture is light, fluffy, and increased in volume. It should resemble a thick, stable marshmallow cream.

Once the cake is completely cool, spread the cream filling evenly over the top. Use an offset spatula for a smooth finish.

Place the cake in the refrigerator while you prepare the ganache, allowing the filling to firm up slightly (about 15-20 minutes).

In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer around the edges (do not boil).

Remove from heat and immediately add the chocolate chips (or chopped chocolate) and corn syrup (if using). Let it sit for 5 minutes without stirring.

After 5 minutes, gently whisk the mixture from the center outwards until it is completely smooth and glossy. Stir in the vanilla extract.

Let the ganache cool slightly at room temperature for 10-15 minutes, stirring occasionally, until it has thickened to a pourable but not watery consistency.

Pour the slightly cooled ganache over the chilled cake, spreading it evenly to cover the cream filling. Allow some to drip down the sides for an authentic look.

Refrigerate the cake for at least 30 minutes (or until the ganache is set) before slicing and serving.