01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Mix cake ingredients per package instructions, pour into pan, and bake for 25 to 30 minutes or until a toothpick inserted into the center comes out clean.
02 - While the cake is baking, combine mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a boil, then lower heat and simmer for 5 to 7 minutes, gently crushing berries. Strain the mixture through a fine sieve to remove seeds and pulp.
03 - After cooling the cake for 10 minutes, poke holes evenly over the surface using a skewer or fork. 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 cold heavy cream with powdered sugar and vanilla extract until soft peaks form.
05 - Spread the whipped cream evenly over the chilled cake. Distribute fresh mixed berries on top.
06 - Refrigerate until ready to serve. Slice into portions and serve chilled.