1. Preheat oven to 325° F. Line ¼ sheet pan with a ¼ sheet pan liner. Lightly brush with oil.
2. In a large bowl, whisk cake mix with eggs, milk, oil, and vanilla until just combined.
3. Pour batter onto pan and spread to edges. Bake 23–25 min, or until a toothpick inserted in the centre comes out clean. Let rest 10 min, then invert cake onto a cooling rack. Let cool completely.
4. Meanwhile, using an electric beater, beat whipping cream until stiff peaks form.
5. In a small, microwave-safe bowl, combine strawberries and dip mix. Microwave, uncovered, on high 1 min, or until hot. Gently mash with a fork.
6. To assemble, cut cake into 3 equal rectangles. Place bottom layer on a platter. Spread ¼ cup whipped cream and 3 tbsp berry mixture over top. Top with next layer and repeat.
7. Spread remaining whipped cream over entire cake and top with remaining berry mixture.
