Preheat oven to 350 degrees. Line a standard size cupcake pan with liners, set aside.
In the bowl of a stand mixer (or a large mixing bowl with a hand mixer), beat the butter and sugar until light and fluffy, about 2-3 minutes. Add the egg whites one at a time, mixing after each addition. Add the vanilla extract, mix to combine.
In a medium mixing bowl, whisk together the cake flour, powdered strawberries, baking powder, baking soda, and salt. With mixing speed on low, gradually alternate adding the dry ingredients along with the sour cream and milk. After all of the ingredients have been added, beat on medium for 15 seconds. Gently fold in diced strawberries.
Using a large cookie scoop (#20 or 3.5 tablespoons), fill each of the liners with the batter. Place in the oven and bake for 18-22 minutes or until the centers are set. You can use a toothpick or cake tester to ensure that they are done. Remove from the oven and allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Meanwhile, to make the frosting, in the bowl of a stand mixer fitted with the whisk attachment (or a large mixing bowl using a hand mixer and the whisk attachment), beat the butter until smooth and creamy, about 1 minute. Add the powdered strawberries and turn the mixing speed on low. Gradually add the powdered sugar along with the raspberry puree, white chocolate, whole milk, vanilla extract, and salt. Turn mixing speed up to high and beat for 3 minutes until fluffy. Chill in refrigerator for 15 minutes before using.
Using a pastry bag fitted with a large piping tip, pipe the frosting onto the cooled cupcakes. Top with a fresh strawberry, if desired.