In a medium bowl add the flour, baking powder, cinnamon, and salt. Beat the butter, and brown sugar until creamy, about 3 minutes. Add the eggs, buttermilk, and vanilla and beat until well incorporated. Combine the milk, baking soda and vinegar. Add this mixture to the batter along with the flour mixture and beat on low speed until completely combined.
Put about 1 tablespoon of batter onto the baking sheet. Repeat making sure to leave about 2 inches in between the cakes.
Bake on 375 degrees for about 10-12 minutes until the cakes begin to brown. Let them cool for a couple minutes on the baking sheet before moving them.
To make the marshmallow cream, beat the marshmallow fluff and butter in a mixer, starting on low, then gradually increasing the speed to medium until the mixture is smooth and fluffy. Reduce the speed to low and add the powdered sugar and vanilla and mix well.
Place finely chopped chocolate into a heat-proof glass or metal bowl. Heat cream until just simmering and immediately pour the warm cream over the chocolate.Let the two sit for a few minutes before stirring. Stir until smooth.
Pair the cakes up with cakes of similar size and turn them over with the flat side up. On half of the cakes, put the marshmallow cream. On the other half, put the cooled chocolate ganache. Bring the two cakes together, but do not squeeze. You may store them in the fridge in an air tight container.