Make the Cupcakes: Preheat oven to 350 degrees F. Line a standard muffin pan with paper cupcake liners.
Sift the cake flour, sugar, baking powder, baking soda, and salt in a large mixing bowl.
In a medium bowl, whisk together ¼ cup of the milk, the sour cream, the egg, and the egg white; set aside.
Beat the dry ingredients on low speed for 15 seconds to combine.
Add the butter, vanilla, and remaining ¼ cup of the milk. Mix on medium speed until the dry ingredients are moistened, about 1 minute. Scrape down the sides and bottom of the bowl.
With the mixer on medium speed, add the wet ingredients in 3 additions, beating for 15 seconds after each addition. Scrape down the sides of the bowl and beat for an additional 15 seconds. It’s okay if there are some lumps, do not overmix!
Using a rubber spatula, gently fold in the chopped strawberries.
Divide the batter evenly between muffin cups. Bake until a toothpick inserted in the center comes out clean, about 20 to 25 minutes. Cool for 15 minutes in the pan, then turn out to a wire rack to cool completely.
Make the Strawberry Meringue Buttercream: Place the strawberries in a food processor and puree until smooth.
Place the egg whites in a heatproof mixing bowl (the bowl of your stand mixer would be ideal, but another heatproof bowl will work). Fill a medium saucepan with 1 to 2 inches of water and place over medium-low heat for a slow simmer.
Add the sugar and whisk together. Place the bowl over the saucepan and whisk constantly until the sugar is completely dissolved and the mixture reaches 160 degrees F on an instant-read thermometer. This can take anywhere from a few minutes up to 10 minutes.
Remove the bowl from the saucepan and place on your stand mixer fitted with a whisk attachment (you can also use a hand mixer if that’s all you have). Whip on medium-high speed until stiff, glossy peaks form, about 10 to 15 minutes. (If you have stiff peaks but your bowl or the meringue is still warm to the touch, place the bowl in the refrigerator for 10 minutes or so to cool down before you add the butter.)
Switch to the paddle attachment and beat on medium-low speed while adding the butter one piece at a time, waiting for each piece to fully incorporate before adding the next.
Once all of the butter has been incorporated, add the vanilla extract and salt. Beat for 30 seconds, stop and scrape down the sides and bottom. With the mixer running, add the strawberry puree and continue to beat until incorporated and silky smooth. The frosting may separate and look curdled, but KEEP BEATING! It will come back together. Once it has all reincorporated, increase the speed to medium high and beat until light and fluffy.
Pipe or slather the frosting on the top of the cupcakes.
Doubling Recipe: If you plan to double this recipe, use ⅓ cup sour cream and 3 whole eggs, and double all of the remaining ingredients.
Mini Cupcakes: This recipe will make 24 mini cupcakes. Use a mini muffin pan and bake at 350 degrees F for 9 to 14 minutes.
9x13-inch Cake: This can be baked in a 9x13-inch pan. Bake at 350 degrees for 28 to 32 minutes.
Layer Cake: Divide the batter between two 9-inch cake pans and bake at 350 degrees for 18 to 22 minutes.
Storing unfrosted cupcakes: Store unfrosted cupcakes for up to 1 day in an airtight container.
Storing frosted cupcakes: Frosted cupcakes can also be stored for up to 1 day on the counter in an airtight container. They also can be stored for up to 3 days in the refrigerator in an airtight container.
Freezing: Freeze cupcakes for up to 2 months in an airtight, freezer-safe container. Thaw overnight in the refrigerator and allow to come to room temperature before serving.
Making ahead: Since they do not last longer than 3 days I would recommend freezing the cupcakes if you plan to make them in advance.