Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: In a large mixing bowl, combine the devil's food cake mix, 1 egg, and 8 tablespoons of melted butter. Mix until well combined and smooth.
Step 3: Pour the cake batter into the prepared 9x13 inch baking pan and spread it evenly.
Step 4: In another bowl, beat together the softened cream cheese, 1 cup of Nutella, 3 eggs, and 1 teaspoon of vanilla until smooth and creamy.
Step 5: Carefully pour the Nutella cream cheese filling over the cake batter in the pan, spreading it evenly.
Step 6: Bake for 40-45 minutes, until the edges are set and the center is slightly gooey.
Step 7: Allow the cake to cool in the pan for at least 15 minutes before slicing. Serve warm or at room temperature.