Preheat your oven to 350 degrees F. Line a standard muffin pan with paper cupcake liners, ensuring easy removal once baked.
In a large mixing bowl, sift together the cake flour, granulated sugar, baking powder, baking soda, and salt. This process aerates the ingredients, contributing to the lightness of the cupcakes.
In another medium bowl, whisk together ¼ cup of the milk, the sour cream, the egg, and the egg white; set aside. These ingredients are crucial for moisture and texture.
Beat the dry ingredients on low speed for about 15 seconds until well combined. This step allows even mixing without overworking the flour.
Add the butter, vanilla extract, and the remaining ¼ cup of milk. Mix on medium speed until the dry ingredients are moistened, which should take about 1 minute. Scrape down the sides and bottom of the bowl to incorporate all ingredients.
With the mixer still on medium speed, add the wet ingredients in three additions, beating for 15 seconds after each addition. It’s okay if the batter has some lumps; overmixing can lead to a dense cupcake!
Using a rubber spatula, gently fold in the chopped strawberries. This adds bursts of flavor that will be delightful in every bite.
Divide the batter evenly between the lined muffin cups, filling each about two-thirds full. Bake until a toothpick inserted in the center comes out clean, usually about 20 to 25 minutes.
Cool the cupcakes in the pan for 15 minutes, then carefully turn them out onto a wire rack to cool completely before frosting.
While the cupcakes are cooling, it's time to prepare the strawberry meringue buttercream. Start by pureeing the fresh strawberries in a food processor until smooth.
Place the egg whites in a heatproof mixing bowl. Fill a medium saucepan with 1 to 2 inches of water and heat over medium-low for a gentle simmer.
Add the granulated sugar to the bowl with the egg whites and whisk together. Place the bowl over the saucepan and whisk constantly until the sugar dissolves completely and the mixture reaches 160 degrees F on an instant-read thermometer.
Once the temperature is reached, remove the bowl from the heat and place it on your stand mixer fitted with a whisk attachment. Beat on medium-high speed until stiff, glossy peaks form, which can take about 10 to 15 minutes.
Switch to a paddle attachment and reduce the speed to medium-low, adding the butter one piece at a time, waiting for each piece to fully incorporate before adding the next.
After all the butter is incorporated, add the vanilla extract and a pinch of salt. Beat for 30 seconds, then scrape down the sides and bottom of the bowl.
With the mixer running, add the strawberry puree and continue to beat until the mixture is silky smooth. If it looks curdled, keep beating; it will come together. Once incorporated, increase the speed to medium-high and beat until light and fluffy.
Finally, pipe or slather the frosting generously on top of the cooled cupcakes. Enjoy!
