01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan. Prepare the cake mix with eggs, oil, and water as directed on the package. Pour the batter into the pan and bake for 25 to 30 minutes or until a toothpick inserted in the center emerges clean.
02 - In a saucepan, combine 2 cups mixed berries, granulated sugar, water, and lemon juice. Bring to a boil over medium heat. Lower the heat and simmer for 5 to 7 minutes, gently mashing berries with a spoon. Strain the mixture through a fine sieve to remove seeds and pulp.
03 - Let the cake cool for 10 minutes. Using a skewer or fork, poke holes throughout the surface at 1-inch intervals. Gradually pour the warm berry syrup evenly over the cake, ensuring full absorption. Allow to cool to room temperature, then refrigerate for a minimum of 1 hour.
04 - Chill a mixing bowl. Whip heavy cream, powdered sugar, and vanilla extract together until soft peaks form.
05 - Evenly spread the whipped cream over the chilled cake. Arrange fresh berries generously over the top.
06 - Return the cake to the refrigerator until ready to slice. Serve chilled.