01 - Preheat the oven to 350°F. Grease a 9x13-inch baking pan. Mix cake ingredients according to package directions, pour into prepared pan, and bake for 25 to 30 minutes, or until a toothpick inserted in the center emerges clean.
02 - Combine 2 cups mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil, then reduce to a simmer for 5 to 7 minutes while gently crushing the berries. Strain syrup through a fine sieve to remove seeds and pulp.
03 - After cooling the cake for 10 minutes, poke holes across the entire surface using a skewer or fork, spacing about 1 inch apart. Gradually pour warm berry syrup evenly over the cake, allowing it to fully absorb. 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 using a hand or stand mixer.
05 - Spread the whipped cream evenly over the cooled cake. Scatter the remaining mixed berries generously over the surface.
06 - Refrigerate until service. Slice and present chilled.