Preheat oven to 350 degrees F. Line a 12-cup muffin tin with paper cupcake liners.
Sift cake flour, baking powder, baking soda, and fine sea salt into large mixing bowl. Whisk to combine. Set aside.
Add unsalted butter and granulated sugar to stand mixer bowl. Using paddle attachment on medium speed, beat the butter and sugar until light and fluffy, about 3 minutes.
Add egg and egg whites to sugar mixture. Beat on low for 1 minute, or until fully incorporated. Scrape down bowl, if necessary.
Add buttermilk, strawberry puree, and vanilla bean paste to small bowl. Stir to combine.
Add half cake flour mixture to stand mixture. Beat on low just until combined. Add buttermilk mixture. Beat on low until fully combined. Add remaining cake flour mixture. Beat just until combined. Avoid over mixing.
Fold in diced fresh strawberries.
Scoop cupcake batter into prepared muffin tin. Bake in preheated oven for 22 minutes, or until a toothpick inserted in the center of a cupcake comes out clean.
Remove muffin tin from oven and place on wire rack to cool. Remove cupcakes from muffin tin and cool completely before frosting.