For the Cupcakes:
Preheat the oven to 350°F (175°C). Line a cupcake pan with 12 liners and set aside.
In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, cream together the softened butter and granulated white sugar using an electric hand mixer on high speed until fluffy, about 2-3 minutes.
Add the egg whites and vanilla extract to the butter mixture, and mix on medium-high speed for 1 to 2 minutes until pale and smooth.
Alternate adding the dry ingredients and the buttermilk to the butter mixture, mixing on low speed, then medium speed, until just combined. Scrape the sides of the bowl as needed. The batter will be thick.
Divide the batter evenly among the 12 liners, filling each two-thirds to three-quarters full.
Bake the cupcakes for 16 to 18 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the pan for 10 minutes before transferring them to a cooling rack.
For the White Chocolate Ganache:
In a double boiler over boiling water, combine the chopped white chocolate, butter, and heavy cream. Melt together, stirring occasionally.
Remove the ganache from the heat and transfer it to a small bowl. Allow it to cool to room temperature, then whip with an electric mixer on high speed until pale and fluffy. Transfer the ganache to a piping bag.
For the White Chocolate Buttercream:
In a large bowl, whip the softened butter and salt with an electric mixer on high speed for 5 to 10 minutes until the butter is pale, fluffy, and doubled in size.
Add the melted and slightly cooled white chocolate, and mix on medium speed until combined.
Gradually sift the powdered sugar into the mixture, ½ cup (65 g) at a time, mixing on low then medium speed until each addition is fully incorporated. Scrape the sides of the bowl as needed. After the final addition, mix on high speed for about 1 minute until the frosting is light and fluffy.
Transfer the buttercream to a piping bag fitted with a decorative tip.
Assembling the Cupcakes:
Once the cupcakes are cool, remove the centers of each cupcake and fill with the white chocolate ganache.
Pipe a generous amount of white chocolate buttercream onto each cupcake.
Serve and enjoy!