1. Preheat your oven to 400 F (200 C). Wash the Russet potatoes thoroughly and pat them dry. Pierce each potato several times with a fork. Place them directly on the oven rack and bake for about 45-60 minutes, or until they are very tender when squeezed. A good visual cue is when the skin looks slightly wrinkled and they yield easily to pressure.

2. Once baked, carefully remove the potatoes from the oven and let them cool for about 10-15 minutes until they are cool enough to handle. Slice each potato in half lengthwise and scoop out the fluffy interior into a large mixing bowl, leaving the skins behind.

3. Add the 1/2 cup melted butter, sour cream, milk, garlic powder, onion powder, salt, and pepper to the scooped potatoes. Using a potato masher or an electric mixer on low speed, mash the ingredients until smooth and creamy. Be careful not to overmix, as this can make the potatoes gummy.

4. Gently fold in 3/4 cup of the shredded cheddar cheese into the mashed potato mixture until just combined. The residual heat from the potatoes will start to melt the cheese, making it wonderfully gooey.

5. Transfer the creamy mashed potato mixture into a 9×13-inch baking dish, spreading it evenly. Sprinkle the remaining 1/4 cup of shredded cheddar cheese over the top of the casserole. Drizzle the extra 2 tablespoons of melted butter over the cheese for a golden, crispy topping.

6. Place the casserole in the preheated 400 F (200 C) oven and bake for 20-25 minutes, or until the cheese is melted and bubbly, and the top is lightly golden brown. If you desire a deeper golden crust, you can briefly place it under the broiler for 1-2 minutes, watching it very carefully to prevent burning.

7. Remove the casserole from the oven and let it rest for a few minutes before serving. Garnish with fresh chopped chives or green onions, if using, for a pop of color and fresh flavor.