Preheat your oven to 350°F (175°C) and grease two 9-inch cake pans with cooking spray or butter.
In a large mixing bowl, combine the white cake mix and strawberry Jell-O.
Add the mashed strawberries, vegetable oil, eggs, water, whole milk, and sour cream to the bowl.
Using a hand mixer or stand mixer, beat the mixture on medium speed for 2-3 minutes, until the batter is smooth and well-combined.
Pour the batter evenly into the two cake pans.
Bake the cakes in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Remove the cakes from the oven and let them cool completely on a wire rack.
While the cakes are cooling, prepare the frosting. In a large mixing bowl, beat the butter and cream cheese together until smooth and creamy.
Gradually add the powdered sugar to the bowl, 1 cup at a time, and beat on low speed until the sugar is fully incorporated.
Add the vanilla extract and mashed strawberries to the bowl, and beat on medium speed for 1-2 minutes, until the frosting is smooth and well-combined.
Once the cakes are cool, place one cake layer on a cake plate or stand.
Spread a thick layer of frosting over the top of the cake layer, using a spatula or frosting knife to spread the frosting evenly.
Place the second cake layer on top of the first, and repeat the frosting process.
Decorate the top of the cake with fresh strawberries or other toppings of your choice.