01 - Prepare and bake the vanilla cake mix according to package directions in a 9x13-inch baking dish. Allow to cool for 15 minutes before proceeding.
02 - Using the handle end of a wooden spoon, poke holes uniformly across the entire cake surface, spacing approximately 1 inch apart to allow sauce penetration.
03 - Combine mixed berries, granulated sugar, water, and lemon juice in a saucepan. Bring to a gentle simmer over medium heat. 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 - Pour the warm berry sauce through a fine mesh sieve to remove seeds if a smoother texture is preferred. Immediately distribute the strained sauce evenly over the 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 heavy whipping cream, powdered sugar, and vanilla extract using an electric mixer 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 artistically on top and finish with mint leaves just before serving.