01 - Prepare and bake the vanilla cake mix according to package instructions in a 9x13-inch baking dish. Remove from oven and allow to cool for 15 minutes.
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 over medium heat. Bring to a gentle simmer. 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 pour the strained sauce evenly over the surface of the cake, ensuring it flows into the poke holes.
05 - Allow the cake to cool completely to 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 on medium-high speed until soft peaks form. Add powdered sugar and vanilla extract, then continue whipping until stiff peaks hold their shape.
07 - Spread the whipped cream evenly over the chilled cake surface. Top decoratively with fresh mixed berries and mint leaves just before serving.