Step 1: Bake the Cake

1. Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish.
2. In a large mixing bowl, prepare the cake batter according to the instructions on the box mix, substituting water with milk for a richer flavor.
3. Pour the batter into the prepared baking dish and bake according to the package instructions, typically 25-30 minutes.
4. Once baked, remove from the oven and let it cool for about 10 minutes.

Step 2: Poke Holes in the Cake

1. Using a fork or skewer, poke holes all over the surface of the cake. This will help the tres leches mixture soak in evenly.

Step 3: Prepare the Tres Leches Mixture

1. In a medium mixing bowl, whisk together evaporated milk, sweetened condensed milk, heavy cream, and vanilla extract until well combined.

Step 4: Soak the Cake

1. Slowly pour the tres leches mixture over the entire cake, ensuring it seeps into the holes.
2. Cover the cake and refrigerate for at least 3 hours (or overnight for best results) to allow the cake to absorb the milk mixture fully.

Step 5: Make the Whipped Topping

1. In a large mixing bowl, beat heavy whipping cream, powdered sugar, and vanilla extract with an electric mixer on high speed until stiff peaks form.
2. Spread the whipped cream evenly over the chilled cake.
3. Sprinkle ground cinnamon on top and garnish with fresh berries if desired.