01 - Bake the cake according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes after removing from the oven.
02 - Using the handle end of a wooden spoon, poke holes evenly across the entire surface of the cake, spacing approximately 1 inch apart to ensure thorough absorption of the berry sauce.
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 - For a smoother texture, press the berry sauce through a fine mesh sieve to remove seeds. Pour the warm sauce generously over the cooled cake, ensuring it flows into the poked holes evenly across the surface.
05 - Allow the cake to cool completely to room temperature, then refrigerate for a minimum of 1 hour to let the berry mixture fully permeate the cake and flavors to develop.
06 - Beat the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form, being careful not to overbeat.
07 - Spread the whipped cream evenly over the chilled cake surface. Top generously with fresh mixed berries and mint leaves just before serving for optimal presentation.