Whisk strawberry milk, egg whites, and vanilla in a bowl. Using a stand mixer fitted with the paddle attachment, mix flour, sugar, baking powder, and salt on low speed until combined. Add butter, 1 piece at a time and mix until only pea-size pieces remain, about 1 minute. Add half of milk mixture, increase speed to medium-high, and beat until light and fluffy, about 1 minute. Reduce speed to medium-low, add remaining milk mixture and beat just until incorporated, about 30 seconds. Give the batter a final stir by hand.