Make the Cupcakes:

1. Preheat oven to 350F and line cupcake pan with cupcake liners.
2. In medium bowl combine dry ingredients: flour, baking powder, baking soda and salt.
3. In microwave safe bowl, melt butter. Then whisk in the sugar, buttermilk, sour cream and vanilla.
4. Add the liquid ingredients to the dry ingredients and mix until combined.
5. In bowl of stand mixer, beat egg whites until soft peaks form. Fold into the batter until combined.
6. Fill cupcake liners ¾ full.
7. Mix sugar and cinnamon topping together and sprinkle ½ teaspoon on top of each cupcake. Reserve remaining for final décor.
8. Bake for 15-18 minutes, or until toothpick inserted comes out with a few crumbs.
9. Allow cupcakes to cool in pans for 5 minutes, then remove to wire racks to finish cooling.

Make the Frosting:

1. Mix butter, brown sugar and vanilla in bowl of stand mixer on medium speed for 2 minutes, until smooth.
2. Reduce mixer speed to low, and slowly add in confectioners’ sugar 1 cup at a time while mixer is running and mix until smooth.
3. Prepare a piping bag fitted with a 1M tip and fill with frosting.
4. Swirl buttercream on top cupcakes and sprinkle with remaining cinnamon-sugar mixture.