3/4 cup diced strawberries (dice into about 1/4-inch pieces)
Diced and whole strawberries for topping (about 1 lb small strawberries)
Frosting: (Double if frosting a double layer cake)
1/2 cup + 2 Tbsp strawberry puree
1/2 cup unsalted butter , nearly at room temperature
1/4 cup salted butter , nearly at room temperature
2 1/2 - 2 3/4 cups powdered sugar
1/4 tsp vanilla extract
Preheat oven to 350 degrees. Sift cake flour into a mixing bowl then add baking powder, baking soda and salt, whisk 20 seconds, set aside.
In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter and granluated sugar until pale and fluffy (occasionally stop and scrape down bowl throughout entire mixing process if not using a paddle attachment that constantly scrapes bowl). Mix in egg then mix in egg whites one at a time. In liquid measuring cup used to measure buttermilk, whisk together the buttermilk, 1/3 cup strawberry puree and the vanilla extract. Working in three separate batches, beginning and ending with flour mixture, add 1/3 of the flour mixture (to the butter mixture) alternating with half of the buttermilk mixture and mixing just until combined after each addition. Fold in the 3/4 cup diced strawberries. Divide batter among 12 paper lined muffin cups, filling each 3/4 full (nearly 1/3 cup in each). Bake in preheated oven 20 - 23 minutes until toothpick inserted into center of cupcake comes out clean. Allow to cool in muffin tin several minutes then transfer to a wire rack to cool completely before frosting.
For the Strawberry Buttercream Frosting:
Add 1/2 cup + 2 Tbsp strawberry puree to a small saucepan. Heat mixture over medium-low heat, and simmer, stirring occasionally until reduced by about 2/3 to 3 Tbsp, about 10 - 14 minutes (measure and if it isn't quite 3 Tbsp then return and continue to simmer, if it's not reduced to correct amount it will make frosting runny). Pour reduced puree into a small bowl, then freeze until cool (stir occasionally for even chilling, it will chill quickly in the freezer since it's such a small amount).
In the bowl of an electric stand mixer fitted with the paddle attachment, whip unsalted and salted butter until very pale and fluffy (nearly white. Stop and scrape down bowl occasionally throughout entire mixing process if not using the paddle attachment that constantly scrapes bowl). Mix in 1 cup powdered sugar, then blend in 3 Tbsp reduced strawberry puree, vanilla extract, and optional red food coloring. Add remaining 1 1/2 cups powdered sugar and continue to mix until frosting is pale and fluffy, adding in an additional 1/4 cup powdered sugar if needed to reach desired consistency.
To assemble cupcakes:
Place a small mound of small diced strawberries on top center of cupcakes. Pipe frosting around and over berries then decorate with a whole small strawberry. Store cupcakes in an airtight container.
To get the strawberry puree, puree strawberries in a food processor then measure out amount needed. For both the puree for cupcakes and frosting it was about 10 medium strawberries but that amount will vary just depending on the size of your strawberries.
I just used the buttermilk substitute, so if you don't have buttermilk on hand simply add a scant 1 tsp lemon juice or white vinegar to a liquid measuring cup, then add in enough milk to reach 1/4 cup. Whisk and let rest 3 minutes.