1. Preheat oven to 350°F. Combine oil and 2 tsp. minced garlic in small bowl. Brush squash halves with garlic oil, and sprinkle lightly with ancho chile powder and coriander. Season with salt and pepper, if desired, and place on large baking sheet.
2. Pulse 2 cups corn kernels in food processor until finely chopped and milky. Set aside.
3. Whisk 1/2 tsp. each coriander and ancho chile powder into cornmeal, along with sugar, baking soda, salt, and cayenne (if using) in medium bowl. Set aside.
4. Whisk together buttermilk and eggs in separate bowl. Whisk in butter, then puréed corn, remaining 1 cup corn kernels, goat cheese, Cheddar, and remaining 2 tsp. garlic. Fold in cornmeal mixture with spatula, then fold in black beans, poblano chile, and green onions.
5. Divide filling among squash halves. Sprinkle each squash with extra Cheddar (if using).
6. Bake squash halves 30 to 45 minutes, or until squash are tender and filling is set. Sprinkle with green onions. Squash can be prepared 24 hours ahead, then reheated 20 minutes at 325°F.