01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan. Combine the cake mix with eggs, oil, and water as directed on the box. Pour into prepared pan and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
02 - While the cake bakes, place 2 cups of berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil over medium heat, then simmer for 5–7 minutes, gently crushing the berries. Strain the mixture through a fine sieve to remove seeds and pulp.
03 - Allow the cake to cool for 10 minutes. Use a skewer or fork to poke holes approximately every inch across the cake. Evenly pour the warm berry syrup over the surface, allowing it to soak into the cake. Let cool to room temperature then refrigerate for at least 1 hour.
04 - In a chilled mixing bowl, whip the cold heavy cream with powdered sugar and vanilla extract until soft peaks form.
05 - Spread the whipped cream evenly over the chilled cake. Top with the remaining fresh mixed berries.
06 - Refrigerate until serving. Cut into portions and serve chilled.