Melt Butter & Add Milk:
If it is for special occasions, we strongly recommend using ¾ lb (341g) - 1 lb (454g) unsalted butter. It tasted super amazing.
Switch Instant Pot to Slow Cook Less Mode. Add half of the room temperature unsalted butter and melt it into the potatoes by stirring gently with a silicone spatula. Add in remaining unsalted butter and repeat this step.
Change Instant Pot to Keep Warm Mode.
Pour hot milk on the mashed potatoes and gently stir it in. Allow the potatoes to absorb the milk. Season with Kosher salt and taste to adjust until desired saltiness. We used approx 4 pinches of coarse kosher salt.