01 - Bake the vanilla 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 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 to a simmer over medium heat. Whisk cornstarch with 2 tablespoons cold water separately, then stir into the berry mixture. Cook for 2-3 minutes while stirring constantly until the sauce thickens.
04 - Pour the warm berry sauce through a fine mesh sieve to remove seeds if a smoother consistency is preferred. Evenly distribute the sauce over the cake, ensuring it fills the poked holes.
05 - Allow the cake to cool completely to room temperature, then refrigerate for at least 1 hour to let the flavors meld and the cake absorb the berry sauce.
06 - Beat the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form.
07 - Spread the whipped cream evenly over the chilled cake surface. Arrange fresh mixed berries and mint leaves on top as garnish before serving.