Preheat the oven to 425 degrees F. Lay refrigerated or homemade pie crust inside a lightly sprayed 9 inch pie pan. Place a piece of parchment paper on top with weighted pie weights or beans so it doesn't puff up. Set on a baking sheet and bake for 15 minutes. Remove paper and weights, bake for an additional 2 minutes. Remove.
Lower oven temperature to 350F. In a bowl mix together mashed potatoes with room temp. butter until smooth, then add other ingredients, mix with mixer until smooth. Cover edges of pie crust with a bit of aluminum foil so it doesn't burn.
Pour filling into pie crust, leave on baking sheet so it sits flat on the rack. Set rack to the middle of the oven and bake for 30 minutes, then remove aluminum foil on crust and continue baking an additional 10-15 minutes or until middle is set and crust is lightly browned.
Remove and transfer to a cooling rack until completely cooled so it firms up. Slice when cooled with whipped cream on top.