To make squash easier to cut, prick squash with a paring knife; microwave each squash on HIGH 4 to 5 minutes. Carefully cut squash in half lengthwise using a sharp chef's knife, discarding seeds and membranes. Line 2 large baking sheets with aluminum foil; lightly grease foil. Arrange squash halves, cut side down, on baking sheets.
Cut off pointed ends of garlic bulbs. Place garlic on a piece of aluminum foil, and drizzle with oil. Fold foil to seal; place on 1 baking sheet with squash.
Place 1 baking sheet on upper oven rack; 1 on lower oven rack. Bake at 350° for 1 hour, switching baking sheet positions after 35 minutes. Let cool 10 minutes.
Scoop out squash from shell, and place in a large bowl. Squeeze garlic cloves from bulbs; mash with a fork.
Melt butter in a small saucepan over medium heat. Stir in half-and-half and next 3 ingredients; bring to a simmer. Remove from heat.
Add herbed butter and mashed garlic to squash. Beat at medium speed with an electric mixer until smooth. Serve hot.
Make Ahead: Prepare squash mash. Cool, cover, and chill up to 3 days. Reheat Butternut Squash Mash loosely covered with heavy-duty plastic wrap in the microwave on HIGH 6 to 8 minutes or until heated thoroughly, stirring after 4 minutes, or bake it in a covered casserole dish at 350° until hot.