Combine potatoes, garlic, rosemary leaves, and broth in Instant Pot®. Season with salt and black pepper, to taste, and stir to combine. Add lid and lock into place. Switch the vent to “Sealing” and set the “Manual” setting to 8 minutes.
When cook time is complete, allow pressure to release naturally for 10 minutes, then manually release any remaining pressure.
Carefully remove lid and add heavy cream. Mash the potatoes with a potato masher or immersion blender until almost smooth, but still slightly chunky. Taste and season with additional salt and black pepper, if desired.
Transfer to a serving bowl and top with butter and serve immediately. Enjoy!