Preheat your oven to 350°F and line a muffin tin with cupcake liners.
In a large mixing bowl, whisk together the cake mix and dry Jell-O powder.
Add egg whites, oil, milk, sour cream, and vanilla. Mix until smooth.
Gently fold in the chopped strawberries.
Scoop batter evenly into cupcake liners, filling each about 2/3 full.
Bake for 16–20 minutes or until a toothpick inserted in the center comes out clean.
Let cool completely before frosting.