Preheat oven to 350°. Line a 12-cup muffin pan with cupcake liners.
Sift together flour, baking powder and salt. Set aside.
Pulse 1 cup strawberries in a small food processor until puréed. 1 cup of strawberries will yield about 2/3 cup of purée. Add a few more strawberries, if necessary, or save extra purée for frosting.
In a small bowl, mix together milk, vanilla, and 2/3 cup strawberry purée; set aside.
Cream butter on medium-high speed of mixer until light and fluffy. Gradually add granulated sugar and continue to beat until well-combined and fluffy. Reduce mixer speed to medium and slowly add egg and egg whites until just blended.
Add half flour mixture, slowly, with mixer on low; mix until just blended. Add milk mixture; mix until just blended. Slowly add remaining flour mixture, scraping down sides of bowl with spatula, as necessary, until just blended.
Scoop batter evenly among prepared muffin cups, filling up 2/3 to 3/4 the way full. Bake for 22 to 25 minutes, be sure to test using a toothpick to make sure they are completely done in the center. Transfer muffin pan to a wire rack and let cupcakes cool completely in pan before icing.