01 - Bake cake according to package instructions in a 9x13-inch baking dish. Allow to cool for 15 minutes.
02 - Using the handle end of a wooden spoon, poke holes evenly across the entire cake surface, 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 cold water and stir into berry mixture. Cook for 2-3 minutes, stirring constantly, until sauce thickens.
04 - Pass berry sauce through a fine mesh sieve to remove seeds if desired. Pour warm sauce evenly over cake, ensuring it fills the poke holes completely.
05 - Let cake cool completely to room temperature, then refrigerate for at least 1 hour to allow flavors to meld and set.
06 - Beat heavy cream with powdered sugar and vanilla extract using electric mixer until stiff peaks form.
07 - Spread whipped cream evenly over chilled cake surface. Top with fresh mixed berries and mint leaves just before serving.