Preheat oven to 350°F. Butter a 12cup muffin tin or line with paper liners and set aside.
In a medium bowl, whisk flour, baking powder and salt, set aside.
To make buttermilk, measure 1 cup of milk to 1 tablespoon of vinegar allow it to sit until it curdles.
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, strawberry pure.
With the mixer on low, add half the flour mixture, alternate with milk until well 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.