01 - Prepare and bake the vanilla cake according to package instructions in a 9x13-inch baking dish. Remove from oven and let 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. Bring mixture to a simmer over medium heat. Whisk cornstarch with 2 tablespoons cold water until smooth, then stir into the berry mixture. Continue cooking for 2 to 3 minutes, stirring constantly, until sauce thickens.
04 - Press the berry sauce through a fine mesh sieve to remove seeds if a smoother texture is preferred. Pour the warm sauce evenly over the cake, ensuring it fills the poke holes completely.
05 - Allow the cake to cool completely to room temperature, then refrigerate for at least 1 hour to let the berry mixture absorb and flavors meld.
06 - Using an electric mixer, whip the heavy cream with powdered sugar and vanilla extract on medium-high speed until stiff peaks form.
07 - Spread the whipped cream evenly over the chilled cake surface. Top generously with fresh mixed berries and mint leaves just before serving.