Preheat the oven to 350 degrees. 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. Then set aside the dry ingredients.

In a large bowl, cream together the butter and granulated white sugar with an electric hand mixer on high speed until fluffy, about 2-3 minutes. (If using a stand mixer, use the paddle attachment.)

Next add in the egg whites and vanilla 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 a little at a time until each has been added completely, mixing on low speed, then medium speed for each addition. Mix just until the batter is combined and smooth, scraping the sides of the bowl as necessary. The batter will be thick.

Divide the batter evenly among the 12 liners until each is two-thirds to three-quarters full.

Bake the cupcakes for 16 to 18 minutes or until a cake tester or toothpick comes out clean from the centers. Let the cupcakes cool in the pan for 10 minutes, then transfer them to a cooling rack to finish cooling.

Make the white chocolate ganache by adding the chopped white chocolate, butter and heavy cream to a double boiler over a pot of boiling water.

Melt everything together, stirring occasionally.

Then remove the ganache from the heat and transfer to a small bowl.

Let the ganache cool to room temperature, then whip with an electric mixer on high speed until pale in color and fluffy. Transfer the ganache to a piping bag and cut a bit of the tip off.

Add the butter and salt to a large bowl. Whip with an electric mixer on high speed for 5 to 10 minutes until the butter is pale, fluffy and double in size.

Pour in the melted and slightly cooled white chocolate and combine on medium speed.

Then sift the powdered sugar into the mixture ½ cup (65 g) at a time. Mix on low, then medium speed, making sure each addition is fully combined before adding the next. Scrape the sides of the bowl as necessary. When the last addition is incorporated, mix on high speed for about 1 minute until the frosting is light and fluffy.

Transfer the frosting to a piping bag fitted with a decorative tip.

Once the cupcakes are cool, remove the centers of each cupcake. Fill each with white chocolate ganache. Then pipe a generous amount of white chocolate buttercream onto each. Then serve!