Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: Prepare the vanilla cake mix according to package directions in a large bowl. Fold in the diced fresh strawberries gently.
Step 3: Pour the strawberry cake batter evenly into the prepared baking pan.
Step 4: In a separate medium bowl, combine the softened cream cheese, melted unsalted butter, powdered sugar, and vanilla extract. Beat with an electric mixer until smooth and creamy.
Step 5: Dollop spoonfuls of the cream cheese mixture over the cake batter in the pan. Use a knife or skewer to gently swirl the cream cheese mixture into the cake batter, creating a marbled effect.
Step 6: Bake for 40-50 minutes, or until a wooden skewer inserted into the center of the cake comes out with moist crumbs attached (not wet batter). The top will appear cracked and golden.
Step 7: Remove the cake from the oven and let it cool completely in the pan on a wire rack. As it cools, the 'earthquake' cracks will become more prominent.
Step 8: Once completely cooled, dust with additional confectioners' sugar if desired before slicing and serving. Store any leftovers in the refrigerator.