Preheat your oven to 175°C (350°F). Line both 8x8-inch pans with parchment paper, leaving an overhang for lifting out the cake.

In a mixing bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.

In a large bowl, beat the sugar and eggs with an electric mixer on medium-high until pale and thick, about 2 minutes. Add milk, oil, and vanilla extract. Beat until combined.

Gradually add the dry ingredients to the wet mixture. Mix on low until just combined. Pour in the hot water slowly, mixing until the batter is smooth.

Divide the batter evenly between the two prepared pans. Smooth the tops.

Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.

Let cakes cool in pans for 10 minutes, then lift out and transfer to a rack to cool completely.

Meanwhile, make the cream filling. Beat the butter until creamy. Add powdered sugar gradually, beating well. Mix in marshmallow cream and vanilla extract. Add heavy cream a little at a time until the filling is light and easily spreadable.

Once cakes are completely cooled, place one cake layer (flat side up) on a board. Spread the cream filling evenly over the top.

Top with the second cake layer, flat side down, pressing gently. Trim edges for neatness if desired.

Cut into 12 rectangular snack cakes.