Preheat oven to 425°. Line a rimmed baking sheet with foil.
Place marrow bones on prepared pan; bake until completely cooked through, 15 to 20 minutes.
In a small microwave-safe bowl, melt butter on low in 30-second intervals, stirring between each, until melted. Remove marrow from bones. Add marrow and any drippings from pan to melted butter; whisk in parsley, thyme, and salt.
Fill a medium bowl with ice; carefully place bowl with butter mixture in bowl with ice. Let chill, whisking frequently, until butter has re-emulsified and solidified. Cover and refrigerate until ready to use.
Recipe by Southern Cast Iron at https://southerncastiron.com/acadiana-uncovered/