01 - Bake the vanilla cake according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes until warm to the touch.
02 - Using the handle of a wooden spoon, poke holes evenly 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 over medium heat. Bring to a gentle simmer. Whisk cornstarch with 2 tbsp cold water until smooth, then stir into the berry mixture. Cook 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 strained sauce over the cake, ensuring it fills the poked holes completely.
05 - Let the cake cool to room temperature, then refrigerate for at least 1 hour to allow the berry mixture to fully absorb and flavors to meld together.
06 - Beat the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form and the cream holds its shape firmly.
07 - Spread the whipped cream evenly over the chilled cake surface. Top with fresh mixed berries and mint leaves just before serving for optimal presentation.