01 - Bake the vanilla cake according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes before proceeding.
02 - Use the handle end of a wooden spoon to poke holes evenly across the entire cake surface, 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, then stir into the berry mixture. Cook for 2-3 minutes while stirring constantly until sauce thickens.
04 - Pass 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 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.