Preheat oven to 350°F. Line a 9×13-inch pan with foil or parchment paper and spray with nonstick cooking spray.: You will feel immediate warmth from the oven as it brings the baking environment up to temperature, and the scent of heated metal will be faint but reassuring. Properly lining the pan ensures the bars release cleanly, which matters for neat presentation, and spraying the liner prevents sticking around the edges. A common mistake is skipping the lining and then struggling to remove the bars; take the extra minute to line and spray for a flawless lift.
Add cake mix, butter and eggs to a large bowl. Stir until combined. You can do this by hand or use a mixer.: When these ingredients meet, the aroma of butter mingles with the sweet dust of cake mix , and the batter will take on a soft, slightly glossy look. Using a mixer yields the smoothest batter quickly, while stirring by hand gives you control to avoid overmixing, which keeps the crumb tender. Watch for streaks of dry mix, and stop as soon as the mixture is uniform; overworking the batter is the typical error that produces chewy bars.
Press batter evenly into prepared pan. Bake until light golden around the edges and no longer glossy, approximately 15-20 minutes. Cool completely before frosting.: Pressing the batter creates an even thickness so the bars bake uniformly, and you should feel slight resistance as the batter firms under your fingertips. During baking you will notice the edges turn a warm, golden color and the glossy sheen fade, signals the interior has set. If you remove the pan too soon, the center can be underbaked and collapse, so trust the visual cues rather than a strict timer.
To make the frosting, beat cream cheese with powdered sugar and vanilla until smooth. Mix on high speed while adding heavy whipping cream, then mix until stiff peaks form.: The texture shifts from dense and slightly tacky to light and aerated as you incorporate the heavy whipping cream . You'll hear the mixer change tone, and the mixture will hold peaks that stand tall when you lift the whisk. This aeration is crucial for a fluffy mouthfeel that contrasts the base. Be careful not to overbeat, as the mixture can separate and become grainy.
Frost bars as desired. Store leftover frosting in the refrigerator for up to 3 days.: Spreading the frosting creates a glossy, cloudlike blanket that invites the first slice, and you should feel a smooth resistance under your spatula as it levels. Chilling leftover frosting preserves its structure and flavor, and returning it to room temperature briefly makes it spreadable again. A frequent oversight is leaving frosting out too long, which can soften too much or shift texture; refrigerate promptly when not using.
Just before serving, top bars with sliced strawberries. Slice and serve.: The final act brings vivid color and a fresh scent; sliced strawberries glisten and release a light berry perfume that wakes the entire dessert. Adding them right before serving keeps juices from soaking into the base and preserves bright texture. If you top too early, juices will penetrate the frosting and make the bars soggy, so time this step close to serving.
Store bars in refrigerator for up to 24 hours once berries are added. Make bars up to 2 days in advance and frosting up to 2 days ahead. Don’t assemble until ready to serve.: Cooling and refrigeration maintain the frosting's structure and prevent spoilage, and you will notice the held-in chill when you slice through the bars. Preparing components ahead reduces stress, but assembling fresh retains the best texture contrast. A common misstep is assembling too far in advance, which sacrifices the pristine look and firm bite of the finished bars.