Preheat your oven to 350°F (175°C). Prepare two 9-inch round cake pans by greasing and dusting them with flour or fitting them with parchment circles.
In a large mixing bowl, combine the boxed white cake mix with the strawberry gelatin powder. Whisk to blend the dry ingredients evenly.
To the dry mixture, add the eggs, milk, vegetable oil, sour cream, mashed strawberries, and vanilla extract. Using an electric mixer on medium speed, beat for approximately 2 minutes until the batter is velvety and uniformly mixed.
Evenly pour the prepared batter into the two pans, smoothing the surface with a spatula.
Place in the oven and bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the baked cakes to cool in the pans for 10 minutes. Carefully remove them and set on wire racks to cool fully before assembling.
Meanwhile, prepare the strawberry frosting. In a separate large bowl, beat the softened butter until it becomes pale and fluffy.
Gradually incorporate the powdered sugar, adding one cup at a time and beating well between each addition.
Stir in the pureed strawberries, vanilla extract, and a pinch of salt. Mix until the frosting is smooth. If the consistency is too runny, add a bit more powdered sugar. If it's too stiff, adjust with a splash of milk.
Once cakes are completely cooled, trim the domed tops if necessary. Position one cake round on a serving plate and spread an even layer of frosting across the top.
Gently place the second cake layer on top, then frost the entire cake—top and sides—until evenly covered.
Optional: Adorn with fresh strawberry slices or whole berries for decoration. Refrigerate for 20 to 30 minutes before slicing to achieve clean, sharp cuts.