01 - Preheat the oven to 350°F. Lightly grease a 9x13-inch baking pan. Prepare the cake mix according to the manufacturer's directions with eggs, oil, and water. Pour into the prepared pan and bake for 25 to 30 minutes, or until a toothpick inserted into the center emerges clean.
02 - While the cake is baking, place berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil over medium heat, then lower heat and simmer for 5 to 7 minutes, mashing the berries gently to release juices. Strain the mixture through a fine sieve to remove seeds and pulp.
03 - Allow the cake to cool for 10 minutes. Using a skewer or fork, poke holes across the cake surface at approximately 1-inch intervals. Slowly pour the warm berry syrup over the cake, ensuring even distribution. Let the cake cool to room temperature, then chill in the refrigerator for at least 1 hour.
04 - In a chilled mixing bowl, beat the heavy cream with powdered sugar and vanilla extract using a hand or stand mixer until soft peaks form.
05 - Spread the whipped cream over the fully cooled cake. Top evenly with the fresh mixed berries.
06 - Refrigerate the assembled dessert until serving time. Slice into portions and serve chilled.