Add brown sugar and cinnamon to the melted butter and mix until combined.
Set aside to cool while you prepare the batter.
Cupcakes
Preheat oven to 375°F and line two cupcake tins with paper liners.
Beat butter and sugar till smooth and combined.
Add in eggs and yolk one at a time, mixing thoroughly between each addition.
Combine the dry ingredients. Alternate adding in the dry ingredients and the milk, starting and ending with the dry ingredients, mixing between each addition until fully combined.
Gently fold the cinnamon mixture into the batter, but do not mix in fully.
Bake for 16-18 minutes till golden brown on top.
Remove from oven and let cool in pan for approximately 5 minutes, then move to a wire rack to cool completely.
Icing
Beat the butter until smooth.
Add the remaining icing ingredients until desired texture.
Pipe or spread icing onto cooled cupcakes and enjoy!
Notes
For extra cinnamon filling, add ¼ cup of brown sugar and ½ tablespoon of cinnamon for every additional tablespoon of butter. Be cautious not to use too much or the cupcakes will crumble.Don't fully mix the brown sugar cinnamon swirl into the Cinnamon Cupcakes batter, just swirl it in enough to disburse it evenly. It should not be fully incorporated.Instead of the marshmallow frosting, try topping with Cinnamon Brown Sugar Frosting or Maple Frosting.