Preheat oven to 350F. Place baking cups onto a baking sheet or line a muffin pan with paper liners.
In a medium bowl, whisk together flour, baking powder and salt.
Puree strawberries in a blender or food processor. Pour into 1/3 cup measuring cup, then combine with milk in a liquid measuring cup. Save the extra pureed strawberries for the frosting.
In the bowl of a standing mixer, beat butter on medium-high until creamy. Gradually add sugar and mix until combined. Add egg and egg whites, mixing on low until combined. Scrape down the sides and bottom of the bowl with a rubber spatula if necessary. Then mix in vanilla.
Alternate mixing in the flour and strawberry/milk mixture, ending with the flour. Mix until combined, scraping down the sides of the bowl if needed.
Evenly pour into baking cups or a muffin pan. Bake for 20 – 25 minutes, until top is golden brown and feels springy. Let cool on wire rack.
For the frosting
Beat butter on medium-high until light and creamy. Reduce speed and add powdered sugar, strawberry puree, vanilla and salt, mixing until combined. Scrape down the sides of the bowl with a rubber spatula as necessary.