Preheat oven to 350.
Butter and flour 2 9" round cake pans.
In bowl cream to your butter and sugars until light and fluffy, about 2-3 minutes.
Mix in your molasses and vanilla.
One at a time add your eggs, mixing well after each addition.
In separate bowl combine your flour, salt, baking soda and cinnamon.
Mix into your wet mixtures.
Once combined, fold in your oats.
Pour batter evenly between the two prepared pans.
Bake for about 20 minutes or until toothpick comes out clean.
Remove from oven and let cool in pan.
Meanwhile to make buttercream beat your butter until light and fluffy.
Add your marshmallow cream and beat for about 2-3 minutes until it is well combined.
Add your powdered sugar and vanilla and mix until combined. You shouldnt need to but you can add 1-2 Tbs of cream or milk for thinner consistency.
Remove cakes from pan and place one on cake stand or serving tray.
Spread 1/2 the frosting mixture on top of cake.
Top with other cake and spread remaining frosting on top of that.
Serve and enjoy!