Instructions

• **Preheat the oven.** Preheat oven to 325°F. Line six cups of a standard 12-cup muffin pan with cupcake liners, spacing them apart.
• **Combine dry ingredients.** In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set aside.
• **Combine milk and sour cream.** In a separate small bowl or measuring cup, stir together the whole milk and sour cream.
• **Cream butter and sugar.** In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter, granulated sugar, and vanilla extract on medium speed for 3–5 minutes, until light and fluffy.
• **Add egg white.** Add the egg white and mix until incorporated, scraping down the sides of the bowl as needed.
• **Combine ingredients.** With the mixer on low speed, add the dry ingredients in three additions, alternating with the milk and sour cream mixture. Begin and end with the dry ingredients. Mix just until combined.
• **Fill cupcake liners.** Divide the batter evenly among the six cupcake liners, filling each approximately two-thirds full. Gently smooth the tops with the back of a spoon.
• **Bake.** Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs. Cool briefly in the pan, then transfer to a wire rack to cool completely before frosting.