Preheat the oven to 400°F.

Remove any loose, papery skin from the garlic bulb. Lay the bulb on its side and use a sharp knife to cut about ¼ inch off the top, exposing as many cloves as possible without cutting away too much of each clove.

Place the garlic bulb cut-side up on two small sheets of aluminum foil and drizzle with olive oil. Wrap tightly and transfer to a small, rimmed baking sheet.

Bake for 40–50 minutes, or until the cloves are tender and lightly caramelized. A knife should slide into the center of a clove with almost no resistance. Set aside to cool.

Add the softened butter to a small mixing bowl. Turn the cooled garlic bulb upside down and squeeze the roasted cloves into the bowl. Add the minced rosemary and mix with a fork until smooth and well combined. Taste and add a pinch of kosher salt, if desired.

For an especially smooth, whipped consistency, use a handheld mixer on medium-high speed to combine the ingredients.

Serve immediately as a spread, or transfer to parchment paper and shape into a log. Wrap tightly, twist the ends to secure, and refrigerate until firm enough to slice.