Melt the butter in a 6-inch cast-iron pan. Once melted, add the pumpkin bread cubes and pour the cream mixture over the top. If you have time, let the stale bread soak up this mixture for a few hours in the fridge. If not, simply cover with a tight-fitting lid and cook over medium low heat for 15 minutes. Once the bottom is starting to look set, carefully slide the pudding out of the pan onto a plate. Carefully and confidently, flip the pudding, uncooked side down, back into the pan. Cover and cook for another 10 minutes, or until bottom is also set.