01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan. Mix the cake batter following the package instructions, utilizing the required eggs, oil, and water. Pour the batter into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted in the center emerges clean.
02 - While the cake bakes, combine the mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil, reduce heat, and simmer for 5 to 7 minutes, gently crushing the berries. Strain the syrup through a fine sieve to remove seeds and pulp.
03 - Allow cake to cool for 10 minutes. Using a skewer or fork, poke holes over the entire surface at 1-inch intervals. Slowly pour the warm berry syrup evenly over the cake, letting it absorb into the holes. Let the cake cool to room temperature, then refrigerate for at least 1 hour.
04 - In a cold mixing bowl, beat heavy whipping cream with powdered sugar and vanilla extract using a hand mixer or stand mixer until soft peaks form.
05 - Evenly spread the whipped topping over the cooled cake. Finish by scattering the remaining fresh mixed berries over the whipped cream.
06 - Refrigerate the completed dessert until ready to serve. Cut into slices and present chilled.