01 - Prepare and bake the cake according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes before proceeding.
02 - Using the handle of a wooden spoon, poke holes evenly across the entire surface of the cake, spacing 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 tbsp cold water and stir into the berry mixture. Cook for 2-3 minutes, stirring constantly, until the sauce thickens.
04 - Pour the warm berry sauce through a fine mesh sieve to remove seeds, if preferred. Evenly distribute the sauce over the cake, ensuring it seeps into the holes.
05 - Let the cake cool completely to room temperature, then refrigerate for at least 1 hour to allow the berry flavor to infuse throughout.
06 - Beat 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. Top with fresh mixed berries and mint leaves just before serving.