#### 1. Preheat the oven:
Preheat your oven to 350°F (175°C). Grease and flour an 8x8-inch baking pan or line it with parchment paper for easy removal.
#### 2. Mix the dry ingredients:
In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set aside.
#### 3. Cream the butter and sugar:
In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2–3 minutes.
#### 4. Add wet ingredients:
Beat in the egg white, sour cream, and vanilla extract until smooth. Scrape down the sides of the bowl as needed.
#### 5. Combine the batter:
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Start and end with the dry ingredients, mixing just until combined. Do not overmix.
#### 6. Bake:
Pour the batter into the prepared pan, spreading it evenly. Bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean.
#### 7. Cool and serve:
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Serve plain, dusted with powdered sugar, or topped with your favorite frosting and garnishes.