Preheat your oven to 375°F (190°C). Place the marrow bones on a baking sheet or in a roasting pan, ensuring they're arranged in a single layer.
Drizzle the bones with a bit of olive oil, which will help them roast evenly and prevent sticking. Season the bones with a pinch of salt and freshly ground black pepper. You can also add minced garlic and fresh herbs for extra flavor if desired.
Roast the bones in the preheated oven for about 20-30 minutes, or until the marrow is soft and easily scooped out with a spoon. The exact time may vary depending on the size of the bones.
Carefully remove the roasted marrow bones from the oven and let them cool slightly. Using a small spoon or knife, scoop out the softened marrow from the bones. The marrow should have a consistency similar to room-temperature butter.
In a mixing bowl, combine the room-temperature unsalted butter with the extracted bone marrow. The butter should also be soft enough to easily incorporate the marrow. Mix the butter and marrow together until they are thoroughly combined. You can use a hand mixer or simply a spatula to do this. The resulting mixture should have a creamy, homogeneous texture.
Taste the butter and adjust the seasoning as needed. Depending on your preference, you can add more salt, pepper, or even a touch of fresh herbs for an extra layer of flavor.