Before starting, set the oven to 350°F and get a 12-cup Bundt Pan ready. You can either apply nonstick cooking spray or use the butter/flour method for greasing.
Next, take a large mixing bowl and combine the cake mix, brownie mix, eggs, water, and oil. Whisk the mixture for about 2 minutes, ensuring that any lumps in the batter are mostly dissolved.
After that, pour the batter into the prepared bundt pan and bake for around 50-55 minutes. However, you should check for doneness at 45 minutes. To check, insert a toothpick and look for a few crumbs, but no wet batter.
When the cake is finished baking, remove it from the oven and let it cool in the pan for up to five minutes. Then, carefully turn the cake out onto a cooling rack and allow it to cool for an additional 30 minutes or so.