Set the oven to 425°F (220°C).

Arrange the marrow bones in a single layer on a baking sheet.

Roast in the preheated oven for 15 minutes, or until the marrow is soft and begins to separate from the bone.

Remove from the oven and allow to cool slightly.

Using a small spoon or knife, carefully scoop the marrow out of the bones into a bowl.

Discard the bones or reserve them for making broth.

Reduce the oven temperature to 325°F (165°C).

In a blender or food processor, combine the extracted marrow, eggs, heavy cream, and sea salt.

Blend until the mixture is smooth and well combined.

Grease four 6-ounce (180 ml) ramekins with butter.

Evenly distribute the custard mixture among the prepared ramekins.

Place the filled ramekins in a baking dish.

Pour hot water into the baking dish until it reaches halfway up the sides of the ramekins, creating a water bath.

Carefully transfer the baking dish to the oven.

Bake for 45 minutes, or until the custards are set but still slightly wobbly in the center.

Remove the ramekins from the water bath and let them cool to room temperature.

Serve warm or chilled, garnished with additional chopped parsley if desired.