Preheat the oven to 350°F. Line a 10×15-inch sheet pan (jelly roll pan) with parchment paper, including up the sides. Spray the parchment with nonstick cooking spray.

In a large bowl, whisk together the cake flour and baking powder. Set aside.

In the bowl of a stand mixer with the paddle attachment, cream together butter, sugar, and vanilla extract until light and fluffy, about 5 minutes. Stop to scrape down the sides of the bowl as needed.

With the mixer on the lowest speed, carefully add in egg whites, one at a time, beating well after each addition, stopping to scrape down the sides of the bowl as needed.

With the mixer still on low, add ⅓ of the flour mixture into the creamed mixture, then half the milk, another third of the flour mixture, the remaining milk, and then the remaining flour mixture.

Mix just until combined. Stop to scrape down the sides of the bowl as needed.

Pour the batter into the prepared pan and bake for 28-30 minutes, or until a toothpick inserted in the center comes out clean. Allow the cake to cool to room temperature on a wire rack.

After the cake has come to room temperature, transfer it to the freezer to chill for 10-15 minutes, or until firm. You can also place it in the refrigerator for 1-2 hours or overnight (covered). Chilling the cake makes it easier to move.