01 - Preheat oven to 350°F. Grease a 9x13-inch baking pan. Blend the cake mix with designated eggs, oil, and water according to package guidance. Pour batter into prepared pan and bake for 25–30 minutes until a toothpick inserted in the center emerges clean.
02 - While the cake bakes, combine berries, granulated sugar, water, and lemon juice in a saucepan. Bring mixture to a boil, then lower heat and simmer for 5–7 minutes, mashing berries gently with a spoon. Strain the mixture through a fine sieve to extract seeded syrup, discarding pulp.
03 - Cool the cake for 10 minutes after baking. Pierce the surface at 1-inch intervals using a skewer or fork. Pour the warm berry syrup evenly over the cake, covering the surface and allowing it to seep into holes. Cool to room temperature, then refrigerate for at least 1 hour.
04 - In a chilled bowl, whip the heavy cream with powdered sugar and vanilla extract until soft peaks form. Avoid overbeating to maintain a light texture.
05 - Spread the whipped cream uniformly over the cold, soaked cake. Distribute the remaining fresh mixed berries over the surface for a vibrant finish.
06 - Refrigerate until serving. Slice into portions and present chilled for optimal texture and freshness.