How to Make It
Preheat oven to 425°. Cut off pointed end of garlic bulb; place bulb on a piece of aluminum foil. Drizzle with oil, and sprinkle with 1/4 tsp. kosher salt. Fold foil to seal. Bake 30 minutes; cool 15 minutes. Squeeze pulp from garlic cloves into a small bowl, and mash with a spoon until smooth.
Place potatoes and remaining 3 Tbsp. salt in a large saucepan with cold water to cover by 2 inches. Bring to a boil over medium heat; boil 25 minutes or until potatoes are tender. Drain and return to pan. Reduce heat to medium-low. Cook, gently stirring often, 5 minutes or until dry. Remove from heat.
Bring butter and cream to a simmer in a small saucepan over medium heat. (Do not boil.) Remove from heat.
Transfer potatoes to a large bowl, and mash with a potato ricer or masher until smooth and free of lumps. Add hot cream mixture, egg yolks, and roasted garlic, and stir until thoroughly combined. Spoon potato mixture into a pastry bag fitted with a star tip. Pipe 3-inch-wide mounds 2 inches apart on 2 parchment paper-lined baking sheets.
Bake at 425° for 20 to 25 minutes or until tops are lightly browned. Serve immediately.