Preheat oven to 425°F.

Line a baking sheet with parchment paper.

Spread the bones, marrow side up, on the baking sheet.

Bake in the preheated oven for 15-20 minutes or until the marrow in the center of each bone is bubbly and soft all the way through.

Let marrow bones cool enough to be handled, then scoop out the marrow into a food processor.

Add the softened butter, salt, rosemary or thyme, and fresh parsley to the food processor.

Blend until smooth.

Use as you would butter and refrigerate leftover.