01 - Bake the vanilla cake mix according to package instructions in a 9x13-inch baking dish. Allow the cake to cool for 15 minutes after baking.
02 - Using the handle end of a wooden spoon, poke holes uniformly across the entire surface of the cake, maintaining approximately 1-inch spacing between each hole.
03 - Combine the mixed berries, granulated sugar, water, and lemon juice in a saucepan. Place over medium heat and bring to a gentle simmer. Whisk the cornstarch with 2 tablespoons of cold water until smooth, then stir into the berry mixture. Continue cooking for 2-3 minutes while stirring constantly until the sauce thickens noticeably.
04 - Pour the warm berry sauce through a fine mesh sieve to remove seeds if a smoother consistency is desired. Evenly distribute the sauce over the warm cake, ensuring it fills the poke holes completely.
05 - Allow the cake to reach room temperature, then refrigerate for a minimum of 1 hour to let the berry infusion set and flavors meld together.
06 - Beat the heavy whipping cream with an electric mixer until it begins to thicken. Add the powdered sugar and vanilla extract, continuing to beat until stiff peaks form and the cream holds its shape firmly.
07 - Spread the whipped cream evenly over the chilled cake surface. Arrange fresh mixed berries attractively across the top and garnish with fresh mint leaves if desired. Serve immediately or keep refrigerated until serving time.