Preheat your oven to 350°F (175°C) and generously grease your mini bundt cake pan with butter.
In a large mixing bowl, combine the dark chocolate cake mix, beaten eggs, melted butter, and water. Stir until a smooth batter forms.
Place the hot fudge into a microwave-safe dish and heat for about 30 seconds, stirring in-between until it becomes smooth and pourable.
Fill each bundt pan mold about ¼ full with the prepared batter. Carefully add approximately 1 tablespoon of the melted hot fudge into the center of each mold.
Fill the molds with the remaining batter, ensuring you completely cover the fudge. The molds should be filled to about ¾ full.
Place the mini bundt cake pan into your preheated oven and bake for approximately 15 minutes.
Remove the bundt cake pan from the oven and allow the cakes to cool for a few minutes. After about 5-10 minutes, carefully invert the pan onto a cooling rack and gently tap to release the cakes.
Drizzle warm peanut butter and additional hot fudge over the cooled bundt cakes before serving.