01 - Prepare and bake the vanilla cake according to package instructions in a 9x13-inch baking dish. Allow the cake to cool for 15 minutes before proceeding.
02 - Using the handle end of a wooden spoon, poke holes evenly across the entire surface of the warm cake, spacing approximately 1 inch apart.
03 - Combine mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring the mixture to a gentle simmer over medium heat. In a small bowl, whisk cornstarch with 2 tablespoons 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 - Pass the thickened berry sauce through a fine-mesh sieve to remove seeds if a smoother texture is preferred. Pour the warm sauce generously over the cake, ensuring it fills the poked holes completely.
05 - Allow the cake to cool completely at room temperature, then refrigerate for a minimum of 1 hour to let the berry infusion set and flavors meld together.
06 - Using an electric mixer, whip the heavy cream, powdered sugar, and vanilla extract together until stiff peaks form and the mixture holds its shape firmly.
07 - Spread the whipped cream evenly over the chilled cake surface. Arrange fresh mixed berries attractively across the top and finish with fresh mint leaves if desired. Serve immediately or keep refrigerated until ready to serve.