Preheat an oven to 450 degrees F (230 degrees C). Prick potatoes several times with a fork and place onto a baking sheet.
Bake in the preheated oven until potatoes are easily pierced with a fork, 50 minutes to 1 hour.
Reduce oven temperature to 350 degrees F (175 degrees C). Butter a casserole dish.
Scoop potato flesh into a bowl; discard skins. Add Cheddar cheese, sour cream, 1/4 cup butter, milk, cream cheese, chives, salt, garlic salt, and pepper. Mash mixture until combined and potatoes are still slightly lumpy. Spread into the prepared casserole dish. Create ridges on the surface with a fork, forming slight peaks.
Bake in the preheated oven until peaks are slightly crisped, about 30 minutes. Pour remaining 1/4 cup butter on top. Continue baking until golden brown, about 15 minutes.