Begin by setting your oven to 350°F (175°C). Lightly grease a 9×13-inch baking dish using nonstick spray or butter to prevent the cake from sticking during baking.

In a large mixing bowl, add the ricotta cheese, granulated sugar, and vanilla extract. Beat the mixture with an electric mixer on medium speed until smooth and creamy. Add the eggs one at a time, mixing thoroughly after each addition to ensure a silky, even consistency. Set this mixture aside for later.

In a separate bowl, prepare the chocolate cake batter according to the box instructions. Combine the cake mix, oil, water, and eggs in a medium bowl. Use a hand mixer or stand mixer to beat the batter until smooth and free of lumps.

Pour the prepared chocolate cake batter evenly into the greased baking dish. Then gently pour the ricotta mixture over the cake batter, spreading it carefully to cover the entire surface. It's fine if the layers mix a little—during baking, the layers will naturally reverse and separate beautifully.

Transfer the dish to the preheated oven and bake for 60 to 70 minutes. The cake is done when the top is firm and the center no longer jiggles. To check doneness, insert a toothpick into the chocolate portion—if it comes out clean, it’s ready. Remove the cake from the oven and allow it to cool completely at room temperature before frosting.

While the cake is cooling, whisk the instant chocolate pudding mix with cold milk in a large bowl. Let it sit for 2–3 minutes until thickened. Once the pudding has set, fold in the thawed whipped topping gently until fully blended into a light, fluffy frosting.

When the cake has cooled completely, spread the pudding frosting evenly over the top using a spatula to smooth it out. Cover the frosted cake with plastic wrap or aluminum foil and refrigerate for a minimum of 4 hours to allow the flavors to develop and the layers to set perfectly.