Preheat the oven to 350°F. Spray a 9x9 inch pan with non-stick spray 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 1/2 cups (280 g) cake flour, 1 1/2 tsp baking powder, 1/4 tsp baking soda, 1/2 tsp 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 a stand mixer fit with a paddle attachment.)
10 tbsp (140 g) unsalted butter, 1 1/2 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.
6 large egg whites, at room temperature , 2 tsp vanilla extract
Add in the buttermilk and the strawberry reduction. Mix on medium speed until combined.
3/4 cup (180 ml) buttermilk, at room temperature, 1/2 cup (120 ml) strawberry reduction from above
Add the dry ingredients to the wet ingredients and mix on low speed until the cake batter is combined. Scrape the sides of the bowl with a rubber spatula as necessary.
Pour the batter into the prepared baking pan.
Bake the cake for 43-46 minutes, or until a toothpick comes out clean from the center of the cake with just a few moist crumbs.
Let the cake cool in the pan on a wire rack for 30 minutes. Carefully pull up on the parchment paper to release the cake from the pan, then continue to cool it on the wire rack until completely cooled.