Mash the warm potato flesh using a masher until mostly smooth.
Add the butter. Stir until melted and fully incorporated.
4 tablespoons salted butter
Mix in remaining ingredients. Add sour cream, half-and-half (or milk), blue cheese, bacon, and ¼ cup of the Parmesan cheese. Mix until creamy.
½ cup sour cream, 6 ounces Point Reyes Blue Cheese, ½ cup half-and-half, 8 slices bacon
Adjust consistency. If needed, add more half-and-half one tablespoon at a time for a softer filling.
Season to taste. Add salt, black pepper, and garlic powder. Stir in green onions, if using.
½ teaspoon kosher salt, ½ teaspoon freshly cracked black pepper, ½ teaspoon garlic powder, ½ cup green onions