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 cake, spacing them approximately 1 inch apart.
03 - Combine berries, sugar, water, and lemon juice in a saucepan. Bring to a simmer over medium heat. Whisk cornstarch with 2 tablespoons cold water and stir into the berry mixture. Cook for 2-3 minutes while stirring constantly until the sauce thickens noticeably.
04 - Press the berry sauce through a fine mesh sieve to remove seeds if a smoother consistency is preferred. Pour the warm sauce evenly over the cake surface, ensuring it fills the poked holes completely.
05 - Let the cake cool completely to room temperature, then refrigerate for at least 1 hour to allow the berry mixture to set and 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 cream holds its shape.
07 - Spread the whipped cream evenly over the chilled cake surface. Top with fresh mixed berries and mint leaves just before serving.