Two 9-inch circular baking pans should be greased and floured.
Turn on oven to 325 degrees. Beat cake mix, buttermilk, eggs, and oil
together on low speed for 30 seconds in a large bowl.
Mix for 2 minutes on medium speed. Spread the batter evenly in the pans.
Bake for 28-32 minutes, or until a toothpick inserted in the centre comes out clean.
Mix marshmallows, sugar, milk, and cornflour in a large pot to make the filling.
To make the filling spreadable, chill it in the fridge for about 45 minutes.