Combine potatoes, leek, and salt in a multi-functional pressure cooker (such as Instant Pot). Add water to cover, about 2 cups. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 10 minutes. Allow about 10 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer's instructions, about 10 minutes. Carefully release remaining pressure using the quick-release method, about 5 minutes. Unlock and remove the lid. Drain potatoes well and return them to the pot.

Select Saute function. Toss potatoes until dried and starting to sizzle, about 3 minutes. Add butter; stir until melted. Add milk. Mash potatoes with the back of a wooden spoon for a rustic mash, or run them through a ricer for a smoother mash. Taste and adjust the salt.