01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Prepare cake mix as directed on the package with designated amounts of eggs, oil, and water. Pour batter into the prepared pan and bake 25 to 30 minutes, or until a toothpick inserted in the center emerges clean.
02 - While the cake bakes, combine 2 cups mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil, then reduce heat and simmer for 5 to 7 minutes, crushing berries gently. 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 cake surface at 1 inch intervals. Slowly pour the warm berry syrup over the cake, letting it soak in. Cool to room temperature, then refrigerate for at least 1 hour.
04 - In a chilled mixing bowl, whip heavy cream with powdered sugar and vanilla extract until soft peaks form.
05 - Evenly spread the whipped cream over the cooled cake. Generously distribute the fresh mixed berries as topping.
06 - Refrigerate until serving time. Slice and serve dessert well chilled.