Preheat oven to 350°F. Line a 12-cup cupcake tin with paper liners. Set aside.
Add the butter and sugar to the bowl of a stand mixer fitted with a paddle attachment or using a hand mixer, beat the butter and sugar together until pale, light and fluffy. Add the eggs, beating to incorporate after each addition. Add the vanilla and beat once more briefly.
In a large bowl, whisk together cake flour, all-purpose flour, baking powder, salt and cinnamon. Add the flour mixture in three additions, alternating with buttermilk, beating just until combined after each addition, starting and ending with the flour. Do not over mix.
Evenly divide the batter among the prepared cupcake liners. Bake in preheated oven for 20 minutes or until a toothpick inserted in the center comes out clean. Remove from oven and transfer cupcakes to a wire rack to cool completely before frosting.
For the Frosting:
Add the butter to a large bowl or the bowl of a stand mixer fitted with a paddle attachment. On medium speed, whip the butter for 5 minutes until very creamy. Add the brown sugar, ground cinnamon, powdered sugar and heavy cream. Mix on low until incorporated then increase the speed until frosting becomes light and fluffy.
Frost cooled cupcakes with brown sugar cinnamon frosting. Dust the tops with additional cinnamon, if desired.