Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Wash, hull, and dry about 1 pound of fresh strawberries. Add strawberries to the bowl of a large food processor and pulse several times until strawberries are finely chopped, but not pureed. Set aside.
1 cup finely chopped fresh strawberries
Whisk flour, baking powder, baking soda, and salt in a large bowl.
3 1/2 cups (494g) all-purpose flour, 1 1/2 teaspoons baking powder, 1 teaspoon baking soda, 1 teaspoon salt
In a separate bowl, mix together granulated sugar, brown sugar, and oil.
1 1/2 cups (300g) granulated sugar, 1/2 cup dark brown sugar, 2/3 cup canola oil
Add sour cream, eggs, and vanilla extract. Mix well.
1/2 cup sour cream, 2 large eggs, 2 teaspoons pure vanilla extract
Add flour mixture and chopped strawberries, then stir well to combine. Batter will be very thick.
Use a medium cookie scoop to drop about 2 tablespoons of batter onto prepared baking sheets, spacing them 2-inches apart.
Bake for 10-12 minutes until the tops bounce back at a touch.
Cool for 10 minutes on a baking sheet, then transfer to a wire rack to cool completely.
Make filling by beating butter, powdered sugar, and vanilla in a large bowl.
1 cup salted butter, 4 cups (480g) powdered sugar, 2 teaspoons pure vanilla extract
Add marshmallow fluff and strawberries, then beat well until combined.
14 ounces marshmallow fluff, 1/3 cup finely chopped fresh strawberries
Transfer filling to a large piping bag and pipe onto the bottom sides of half of the cake shells. Top with another shell, then refrigerate until ready to serve.