Preheat oven to 350 F and line a regular-sized cupcake tin with paper liners.
In a small food processor, blend 3 large strawberries until pureéd. This should yield 1/4 cup. Chop remaining 3 strawberries into small pieces. Set both the pureé and chopped strawberries aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In the bowl of a stand mixer (or using a hand mixer), beat butter and sugar on medium speed until light and fluffy, about 4 minutes. Add vanilla, egg yolk and egg whites. Beat just until combined, about 1 minute. Scrape down the sides and bottom of the bowl if necessary.
With the mixer on low, alternate adding the flour mixture and buttermilk, beginning and ending with flour. Mix just until combined (avoid over mixing).
Using a rubber spatula, stir in the strawberry pureé and chopped strawberries by hand. Use this time to scrape down the sides and bottom of the bowl if necessary.
Spoon batter into prepared cupcake tin, filling each cup until 2/3 full. Bake at 350 F for 20 minutes, or until a toothpick inserted in the center comes out clean. Cool cupcakes in the tin for 5 minutes then transfer to a wire rack to cool completely. Prepare frosting.