Bake the potatoes at 400 degrees for 45-60 minutes or until fork tender. Let cool to room temperature.
Roughly chop the potatoes and add to a large bowl with the sour cream, mascarpone, green onion, butter, salt, pepper, and 1 cup shredded cheddar. Mix until well combined. Spread into a medium baking dish and top with the remaining ½ cup cheddar.
If cooking immediately, bake in the preheated oven for about 15-20 minutes or until the cheese is melted. If saving for later, cover and transfer to the fridge. When ready to bake, uncover and bake at 350 degrees for 30-35 minutes or until warmed through and the cheese is melted.
Remove from heat and top with the bacon and more green onion if desired.