01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Prepare cake mix according to package instructions using provided eggs, oil, and water. Pour batter into pan and bake for 25–30 minutes or until a toothpick inserted in the center comes out clean. Allow to cool for 10 minutes.
02 - Combine 2 cups berries, granulated sugar, water, and lemon juice in a saucepan. Bring mixture to a boil over medium heat, then lower to a simmer. Cook for 5–7 minutes, gently crushing berries until juices release. Strain the syrup through a fine sieve to remove seeds and any pulp.
03 - Use a skewer or fork to poke holes evenly across the cake, spacing about 1 inch apart. Gradually pour warm berry syrup over the entire cake surface, letting it absorb fully. Allow cake to reach room temperature, cover, and refrigerate for at least 1 hour.
04 - In a chilled mixing bowl, beat cold heavy cream with powdered sugar and vanilla extract using a hand mixer or stand mixer. Whip until soft peaks form.
05 - Spread whipped cream evenly across the surface of the chilled, soaked cake. Top generously with 2 cups fresh mixed berries. Refrigerate until serving. Slice and enjoy cold.