Preheat oven to 350°F. Butter a 12 cup muffin tin or line with paper liners and set aside.
In a medium bowl, whisk flour, baking powder and salt, set aside.
In a stand mixer with the paddle attachment, beat butter on medium-high speed for 2-3 minutes, until light and fluffy. Scraping the bowl as needed. Gradually add the sugar and beat until well combined. Reduce the mixer to low and add the egg, egg whites, and vanilla extract.
With the mixer on low, add half the flour mixture, mixing until just combined. Add the milk, mixing until just combined. Add the remaining flour and mix until just combined, scraping the sides of the bowl as necessary. Using a rubber spatula, carefully fold in the chopped strawberries.
Divide the batter evenly among the prepared muffin cups. Bake for 18-20 minutes, until a toothpick inserted into the center of the cupcakes comes out clean. Transfer the muffin tin to a wire rack to cool completely before icing.