01 - Prepare the vanilla cake mix according to package directions and bake in a 9x13-inch baking dish. Allow to cool for 15 minutes after removing from the oven.
02 - Using the handle end 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. Meanwhile, whisk cornstarch with 2 tablespoons cold water until smooth, then stir into the berry mixture. Cook for 2-3 minutes while stirring constantly until the sauce thickens.
04 - Strain the thickened 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 poke holes completely.
05 - Allow the cake to cool completely to room temperature, then refrigerate for at least 1 hour to let the berry mixture absorb and flavors meld together.
06 - Beat the heavy whipping cream with powdered sugar and vanilla extract using an electric mixer until stiff peaks form, being careful not to overbeat.
07 - Spread the whipped cream evenly over the chilled cake surface. Top with fresh mixed berries and mint leaves if using. Serve immediately or keep refrigerated until ready to serve.