Pepper Jack Mashed Potatoes

Name: Pepper Jack Mashed Potatoes
Ingredients: 1 1/2 sticks salted butter, plus more for the baking dish; 5 lb. pounds russet potatoes, peeled and cut into 2-inch chunks; 6 garlic cloves, crushed and peeled; 1 1/2 tsp. kosher salt, plus more to taste; 4 oz. cream cheese, at room temperature; 3/4 c. half-and-half; 1/2 tsp. chili powder, plus more for sprinkling; 1/2 tsp. cayenne pepper, plus more to taste; 8 oz. pepper jack cheese, shredded (about 2 cups); Thinly sliced scallions, for garnish