01 - Prepare cake batter according to package instructions and bake 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 evenly across the entire cake surface, spacing approximately 1 inch apart.
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. Cook for 2-3 minutes while stirring constantly until the sauce thickens noticeably.
04 - Pass the berry sauce through a fine mesh sieve to remove seeds if a smoother consistency is preferred. Pour the warm sauce evenly over the cooled cake, ensuring it fills the poked holes completely.
05 - Refrigerate the cake for at least 1 hour to allow the berry mixture to fully absorb and the flavors to meld together.
06 - Beat heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form and the mixture holds its shape.
07 - Spread the whipped cream evenly over the chilled cake surface. Arrange fresh mixed berries and mint leaves decoratively on top before serving.