Preheat the oven to 350℉ (175 ℃). Grease a 9×9 inch 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 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 the bowl of a stand mixer fit with a paddle attachment.) 10 tbsp (140 g) unsalted butter, softened, 1 1/2 cups (300 g) granulated white sugar
Then add in the egg whites, vanilla, and coconut extract and mix on medium speed until pale in color and smooth, about 1 minute.
6 egg whites, at room temperature, 2 tsp vanilla, 2 tsp coconut extract
Add in the coconut milk and sour cream and mix on medium speed just until combined.
1 cup (240 ml) canned, full fat coconut milk (unsweetened), at room temperature , 1/2 cup (122 g) sour cream, at room temperature
Add in the dry ingredients a little at a time. Mix on low then medium speed just until the batter is combined and smooth, scraping the sides of the bowl as necessary.
Pour the batter into the prepared pan. Bake the cake for 40-45 minutes or until a light golden color and a toothpick comes out clean from the center.
Let the cake cool in the pan for 30 minutes, then pull up on the parchment paper to release the cake from the pan and continue cooling the cake on a wire rack.