Preheat oven to 350 degrees. Grease and flour 2 (9-inch) cake pans and set aside.
2.
Sift flours, sugar, cocoa powder, cinnamon, baking soda and salt into a large bowl. Stir in butter, eggs, milk and vanilla extract. Mix with a wooden spoon or rubber spatula until smooth.
3.
Pour batter into prepared pans and bake for 50-55 minutes or until a toothpick inserted into the center comes out clean. Let cakes cool in pans on wire racks before inverting to remove. Cool completely before filling layers and icing cake.
4.
While cake is cooling, make icing: Place chocolate chips into a large bowl. Heat heavy cream until hot but not boiling. Pour cream over chocolate chips and stir until melted.
5.
Add the butter to warm chocolate and stir until melted.
6.
Cover bowl with plastic wrap and refrigerate for about 1 hour or until ganache is the consistency of spreadable icing. Stir the ganache a little to loosen it.
7.
Place cake layers on a flat surface. Spread marshmallow fluff onto one of the layers and top with remaining cake layer. Ice entire cake with ganache.
Tips
Decorate the outside of the cake with whole graham crackers instead of crushed.