Preheat oven to 350 degrees and line cupcake pan with liners. Set aside.
In a medium bowl, whisk (or sift) together the dry ingredients: cake flour, baking powder, and salt. Set aside.
In a large bowl, cream together butter and granulated sugar. Add vanilla and beat until incorporated. Add egg whites and beat until light and fluffy, about 1 minute.
Add flour mixture to butter mixture in thirds, alternating with the milk, until all ingredients are incorporated and batter is smooth. Do not overmix.
Pour batter into cupcake liners, filling each 2/3 full. Batter will fill 15 cupcake liners. Bake in preheated oven for 16-18 minutes, or until a toothpick comes out clean.
Remove pan from oven and allow to cool for 5 minutes before transferring cupcakes to a wire rack to finish cooling.
In a saucepan over medium heat, add strawberries, sugar, lemon juice, and cornstarch. Stir to combine.
Bring to a boil. Reduce heat to medium-low and cook for 4-5 minutes, or until mixture has thickened. Remove from heat and allow to cool.
Gently core the center of each cupcake (about a 3/4" circle) and fill with cooled strawberry filling. Be careful not to core through the bottom of your cupcake. Leave about 1/2" on the bottom so your filling doesn't fall through when unwrapped.
Whipped Buttercream Frosting
In a large bowl, add room temperature butter, beat until creamy, about 1 minute.
Add confectioner's sugar and slowly mix until ingredients are fully combined. Then, whip at high speed for 2-3 minutes, or until light and fluffy.
Add vanilla extract. Then, add heavy cream and whip for an additional 3-4 minutes, or until light and fluffy. Add additional heavy cream as needed to reach desired consistency.
Pipe or spread frosting with a knife onto cupcakes and serve.
For best results, store in refrigerator until ready to serve. Let cupcakes set at room temperature for 30 minutes before serving. Cupcakes will keep for up to 3 days in refrigerator.
Frosting will make enough to frost about 16 cupcakes with high, dramatic piped frosting, or 24 cupcakes spread with a knife, a 9x13 sheet cake, or an 8-9 inch two layer cake.
Butter melts around 90 to 95 degrees, and so will your frosting. If your location is nearing this temperature, I'd recommend refrigerating your baked goods until an hour before you're ready to serve them. Refrigerating your frosted baked goods will not hurt them, but you will want to give them some time to come to room temperature before serving.