Preheat the oven to 350° F. Line two cupcake pans with paper liners. Set aside.
In a medium mixing bowl, combine the cake flour, baking powder and salt. Whisk together and set aside.
Add the butter to the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed for 3 minutes, until light and creamy in color. Scrape down the sides of the bowl and beat for one more minute.
Add the sugar to the butter mixture, ¼ cup at a time, beating 1 minute after each addition. Mix in the eggs one at a time until incorporated. Scrape down the sides of the bowl after each addition. Combine the buttermilk and the vanilla extract in a liquid measuring cup. With the mixer on low speed, add the dry ingredients alternately with the wet ingredients, beginning and ending with the dry ingredients and mixing just until incorporated. Scrape down the sides of the bowl and mix for 15 seconds longer.
Divide the batter between the prepared paper liners, filling each about 2/3 of the way full (you will likely have batter left over after filling 24 wells.I managed to make a full 3 dozen cupcakes) Bake 18-22 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in the pans 5-10 minutes, then transfer to a cooling rack to cool completely. Replace paper liners and bake remaining batter if desired. Frost cooled cupcakes as desired.
To make the vanilla buttercream frosting: Cream the butter until light and fluffy. Add in your powdered sugar 1/2 cup at at time with mixer on low. Cream until combined. With mixer off, add your heavy cream and vanilla extract and then mix on low until combined. Stir in your fluff. Then mix on medium to medium high speed for 8 minutes until light and fluffy. Use right away on cooled cupcakes. If not using right away, then store in a closed container in the refrigerator. You will need to mix the frosting again when ready to use to bring it back to that light and fluffy texture!
To frost the cupcakes, I used an open circle pastry tip (ateco #809). After frosting, I topped each with a half of a rice krispie treat square