01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Mix cake batter using the packaged cake mix, eggs, oil, and water per manufacturer's instructions. Pour batter into prepared pan and bake for 25–30 minutes or until a toothpick inserted in the center comes out clean.
02 - While cake is baking, stir together 2 cups mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring mixture to a boil, then reduce to a simmer and cook 5–7 minutes, gently crushing berries as they soften. Strain through a fine sieve to remove seeds and pulp.
03 - Allow cake to cool for 10 minutes. Using a skewer or fork, poke holes across the surface at 1-inch intervals. Slowly pour the warm berry syrup evenly over the cake to ensure thorough absorption. Let cool to room temperature, then refrigerate for at least 1 hour.
04 - Whip the cold heavy cream in a chilled mixing bowl with powdered sugar and vanilla using a hand mixer or stand mixer until soft peaks form.
05 - Spread whipped cream evenly over the chilled cake. Arrange remaining fresh mixed berries over the top.
06 - Chill until ready to serve. Cut into portions and serve cold.