Make the cupcakes:
Preheat the oven to 350 degrees F. Line a standard 12-cup muffin pan with paper liners.
Puree the strawberries in a food processor or blender. You should get about 1/2 cup puree; if not, puree a few more strawberries to yield 1/2 cup. Set aside.
In a bowl, sift together the flour and baking powder. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together on medium-high for 4 minutes. Add the egg whites and beat for 1 minute on medium, stopping and scraping down the sides of the bowl as necessary. Add the strawberry puree and vanilla extract and mix on medium for 30 more seconds.
In five portions, alternately add the flour mixture and milk to the mixer bowl, beginning and ending with the flour and mixing well after each addition, about 20 to 30 seconds. Fill the liners halfway with batter and bake for 20 minutes, or until a toothpick inserted in the center of a cupcake comes out clean. Let cool in the pan for 5 minutes, then gently release the cupcakes onto a wire rack to cool completely.
Make the strawberry frosting:
Puree the strawberries to yield 1/2 cup. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter for 4 minutes, or until light and fluffy. Add the strawberries and mix. (The mixture may seem curdled, but will come together with the addition of the sugar.) Sift the confectioners' sugar into the bowl 1 cup at a time, mixing well after each addition. Stir in the vanilla extract. Set aside.
Using a teaspoon, gently scoop out a spoonful of cake from the center of each cupcake. Fill the cavity with frosting using a pastry bag or plastic bag with the corner snipped off. Replace the scooped-out cake piece. Frost the top of each cupcake and top with a strawberry slice.
Reprinted with permission from Sweet Home by Rebecca Miller, Kyle Books copyright (c) 2012