Preheat oven to 350 degrees. Gather your ingredients so that you have everything close at hand on the counter or table.
Prepare the Strawberries
Using a potato masher or fork, mash the fresh or frozen strawberries. With a food processor or blender, puree the frozen strawberries for the frosting. Set aside until ready to use.
Make the Strawberry Cake
In a large mixing bowl, mix together on low speed the white cake mix, strawberry gelatin, large eggs, vegetable oil, and buttermilk until combined, about 1 minute. Mix an additional 3 minutes on medium speed.
Using a rubber spatula, fold in the prepared mashed strawberries.
Pour cake batter into a greased oblong cake pan, spreading to edges. Tap pan once to remove bubbles.
Bake in preheated oven for 30-35 minutes until cake springs back in center when touched. Allow to cool.
Prepare the Strawberry Frosting
Combine powdered sugar, pureed strawberries, and softened butter in a medium bowl. Beat together until smooth and creamy.
Spread onto cooled cake. Serves 15-18.
Notes
*Unless otherwise indicated, all butter on this recipe site is salted.