Step 1: Puree 1 lb of fresh strawberries in a blender, then simmer in a small saucepan over medium-low heat for 15-20 minutes until reduced to about 1/2 cup. Let cool completely.
Step 2: Preheat your oven to 350°F (175°C) and line a standard 12-count muffin tin with paper liners.
Step 3: In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt. Set aside.
Step 4: In a large bowl or stand mixer, cream the softened butter and granulated sugar together on medium-high speed for 3-5 minutes until very pale and fluffy.
Step 5: Add the egg and egg whites one at a time, beating well after each addition, then mix in the vanilla extract and sour cream.
Step 6: On low speed, alternate adding the dry ingredients and 1/4 cup of the strawberry reduction to the batter, starting and ending with the dry ingredients. Do not overmix.
Step 7: Fill each cupcake liner about 2/3 full. Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Step 8: Allow cupcakes to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before frosting with strawberry buttercream made using the remaining reduction.