Add butter and lobster shells to a medium sauce pan over medium-low heat. Bring to a bare simmer, and then turn to low. Let simmer 30 minutes. Transfer to a large bowl and let cool to room temperature, stirring occasionally Preheat oven to 450°F. Place marrow bones on a sheet pan lined with foil, standing up. Roast bones for 15-20 minutes in oven. Remove marrow bones from the oven and let cool 5-10 minutes or until cool enough to handle. Using a butter knife or other small blade to scrape the marrow from the bones into the butter. Add the parsley, salt and pepper to the butter mix, then spin in the food processor for 30 seconds. Refrigerate. Let soften at room temperature 15-20 minutes before serving.