Heat the oven to 350°F (175°C). Thoroughly grease a 10- to 12-cup Bundt pan, paying special attention to the center tube and every deep groove, then dust it with cocoa powder or flour.

Whisk the dark chocolate cake mix and instant chocolate pudding mix together in a large bowl. Add the eggs, sour cream, oil, and water, then beat for 1–2 minutes until smooth and fully combined.

Fold in the chocolate chips with a spatula so they are distributed throughout the thick batter. Pour the batter into the prepared pan and smooth the surface.

Bake for 50–60 minutes, or until a skewer inserted into the thickest part comes out with a few moist crumbs but no wet batter. Avoid baking until the skewer is completely dry, because that can make the cake less tender.

Cool the cake in the pan for 10–15 minutes, then carefully invert it onto a wire rack. Let it cool completely, which takes at least 1 hour before glazing and serving.

For the ganache, heat the heavy cream until steaming but not boiling. Pour it over the finely chopped dark chocolate, let it stand for 2–3 minutes, and stir until smooth; mix in the butter if using.

Let the ganache thicken slightly, then pour it over the completely cooled Bundt cake. Slice and serve once the glaze clings to the ridges.