01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Prepare cake mix following package instructions using listed eggs, oil, and water. Spread batter into prepared pan and bake 25–30 minutes, or until a toothpick inserted in the center comes out clean.
02 - While cake is baking, combine 2 cups mixed berries, granulated sugar, water, and lemon juice in a saucepan over medium heat. Bring to a boil, then reduce heat and simmer 5–7 minutes, gently crushing berries. 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 evenly over the entire surface. Slowly pour warm berry syrup over cake, ensuring it soaks in throughout. Cool completely to room temperature, then refrigerate for at least 1 hour.
04 - In a chilled mixing bowl, whip cold heavy cream with powdered sugar and vanilla extract until soft peaks form.
05 - Spread whipped cream topping evenly over the cooled cake. Arrange the remaining fresh mixed berries generously over the surface.
06 - Keep refrigerated until ready to serve. Slice and present the cake chilled for optimal flavor and texture.