For the Cupcakes:
2¾ cups all-purpose flour
½ cup cake flour
1 tablespoon baking powder
1 teaspoon salt
1 cup unsalted butter, at room temperature
2¼ cups granulated sugar
1½ teaspoons vanilla extract
1 egg white
1 cup whole milk
2 cups finely chopped fresh strawberries
For the Strawberry Meringue Buttercream:
1½ cups fresh strawberries (about 8 ounces), rinsed, hulled and coarsely chopped
4 egg whites
1¼ cups granulated sugar
1½ cups unsalted butter, cut into tablespoons, at room temperature
1. Make the Cupcakes: Preheat oven to 350 degrees F. Line standard muffin tins with paper liners.
2. Whisk together both flours, baking powder and salt; set aside.
3. With an electric mixer on medium-high speed, cream together the butter, sugar and vanilla until pale and fluffy, about 3 minutes. Add the eggs and egg white, on at a time, beating until each is fully incorporated, scraping down the sides of the bowl as needed. Reduce the mixer speed to low and add the flour in two batches, alternating with the milk, and beating until well combined. Use a rubber spatula to gently fold in the chopped strawberries by hand.
4. Divide the batter evenly among the lined cups, filling each three-quarters full. Bake until golden and a cake tester inserted in centers comes out clean, 25 to 30 minutes. Transfer to pans to wire racks to cool for 15 minutes; turn out the cupcakes onto the rack and let cool completely. Cupcakes can be stored up to 1 day at room temperature in airtight containers before frosting.
5. Make the Strawberry Meringue Buttercream: Puree the strawberries in a food process or blender; set aside.
6. Combine the egg whites and sugar in the heatproof bowl of a standing electric mixer set over a pan of simmering water. Whisk constantly by hand until the mixture is warm to the touch and the sugar has dissolved (the mixture should feel completely smooth when rubbed between your fingertips).
7. Attach the bowl to the mixer fitted with the whisk attachment. Starting on low and gradually increasing to medium-high speed, mix until stiff (but not dry) peaks form. Continue mixing until the mixture is fluffy and glossy, and completely cool (test by touching the bottom of the bowl), about 10 minutes.
8. Reduce the mixer speed to medium-low and add the butter a few tablespoons at a time, mixing well after each addition. Once all the butter has been added, scrap down the sides of the bowl with a spatula and switch to the paddle attachment; continue beating on low speed until all air bubbles are eliminated, about 2 minutes. Add the strawberry puree and beat until combined. Stir the frosting with a rubber spatula until the frosting is smooth. Keep the buttercream at room temperature if using the same day, or transfer to an airtight container and refrigerate up to 3 days or freeze up to 1 month. Before using, bring to room temperature and beat with paddle attachment on low speed until smooth again, about 5 minutes.
9. Decorate the Cupcakes: Spread or pipe the buttercream onto each cupcake. Store cupcakes in an airtight container at room temperature for up to 1 day, or refrigerate for up to 3 days.