Preheat oven to 350F and line a muffin tin with paper wrappers. Set aside.
Place strawberries in a food processor and pulse several times until strawberries are finely chopped. Line a plate with paper towels and place strawberries in an even layer. Blot strawberries dry with paper towels to remove excess moisture.
Place egg whites in a large bowl and beat to soft peaks with an electric mixer (or a stand mixer fitted with whip attachment). Set aside.
Place butter and sugar in a large bowl (or a stand mixer fitted with paddle attachment) and use an electric mixer to beat on medium speed 2-3 minutes until smooth and creamy. Add vanilla and beat until combined.
In a medium bowl, combine flour, cornstarch, baking powder, and salt. Working in increments, fold 1/3 flour mixture and 1/3 of milk into butter-sugar mixture until smooth and repeat 2 more times until all flour and milk has been added.
Add strawberries and whipped egg whites to batter and very gently fold until batter is just combined.
Fill muffin cups 3/4 full of batter and smooth tops. Bake cupcakes at 350F 20-25 minutes until a toothpick inserted in center of cupcake comes out clean. Cool cupcakes completely on a wire cooling rack.