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.
Pureé strawberries in the food processor. Transfer to a medium saucepan and simmer over medium-low heat stirring occasionally. Simmer until reduced to no more than 3 tablespoons of concentrated pureé. Transfer to a small bowl and cool completely.
In the bowl of a stand mixer (or using a hand mixer), beat butter on medium-high speed until light and fluffy, about 2 minutes.
Add 1 cup confectioners' sugar and beat on medium speed just until combined. Add vanilla and strawberry pureé concentrate and mix just until combined. Add remaining confectioners' sugar and beat until combined.
Transfer frosting to a piping bag fitted with desired tip. Frost cupcakes and garnish with a strawberry if desired. Store cupcakes in the refrigerator for up to 3 days.
*Nutritional information is an estimate. Please consult a doctor or nutritionist for the most accurate nutritional info.