Preheat the oven to 350 degrees. Grease a 9x9 pan and line it with parchment paper.
In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Then set aside the flour mixture.
2 ½ cups (280 g) cake flour, spooned and leveled, 1 ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
Add the softened butter and granulated white sugar to a large bowl. Cream together for 2 minutes with an electric mixer. (You can also use the bowl of a stand mixer fit with a paddle attachment.) 10 tablespoon (140 g) unsalted butter, 1 ½ cups (300 g) granulated white sugar
Then add in the egg whites and vanilla and mix on medium speed until pale in color and smooth, about 1 minute.
4 egg whites, at room temperature, 1 tablespoon vanilla
Add the dry ingredients and buttermilk to the wet ingredients a little at a time, mixing on low speed until the cake batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.
1 cup (240 ml) buttermilk, at room temperature
Pour the batter into the prepared baking pan.
Bake the cake for 33-36 minutes or until a cake tester comes out clean from the center.
Let the cake cool in the pan for 30 minutes. Then pull up on the parchment paper to remove the cake from the pan. Let it finish cooling on a wire rack until completely cool.