Preheat oven to 375 degrees F. Spread frozen strawberries on a parchment-lined baking sheet. Roast 25 to 30 minutes, stirring once, until thick and jammy. Cool completely, then blend or mash smooth. Reserve 3/4 cup for the batter and set the remainder aside for filling.

Reduce oven to 350 degrees F. Line cupcake pans with liners. Combine cake mix, egg whites, oil, buttermilk, sour cream, vanilla, and 3/4 cup cooled strawberry puree in a large bowl. Beat on medium speed 2 to 3 minutes until smooth and fluffy. Divide evenly among liners, filling each about 2/3 full.

Bake 18 to 22 minutes until a toothpick inserted in the center comes out clean and tops spring back lightly. Cool in pans 5 minutes, then transfer to wire racks to cool completely.

Using a small spoon, melon baller, or cupcake corer, remove a shallow core from the center of each cooled cupcake. Add about 1 teaspoon of remaining strawberry puree into each core, then top with several pieces of diced fresh strawberry. Do not overfill.

In a chilled mixing bowl, beat cold heavy cream, powdered sugar, vanilla, pudding mix, and cornstarch on medium-high until stiff peaks form, about 3 to 4 minutes. Keep refrigerated until ready to use.

Pipe or spread frosting generously onto each cupcake. Top with a strawberry slice or half. Refrigerate uncovered 30 minutes to set, then cover loosely and refrigerate overnight.

Remove from refrigerator 20 to 30 minutes before serving for best texture and flavor.