Peel the potatoes and cut them into halves or quarters, depending on the size. You want them to be about the same size to ensure an even cook.
Transfer the cut potatoes into the Instant Pot. Add the broth until the potatoes are just covered. If you need more liquid, you can use water.
Secure and lock the lid into place. Position the valve to sealing, and cook the potatoes on high pressure for 8 minutes. (Proceed to the next step while the potatoes cook.) When the timer goes off, do a quick release and drain the potatoes, then return them to the Instant Pot.
While the potatoes are cooking, melt the butter over medium-low heat on the stove in a small saucepan. When the butter is melted, add the grated garlic and chopped fresh herbs. Stir often to keep the butter from separating. If the butter separates, heat it again and keep stirring.
Give the cooked potatoes a quick mash with a potato masher or with the beaters from your handheld mixer, depending on what you're using. Add the butter, milk, Greek yogurt, salt, and pepper, and mash/beat the potatoes until creamy. (Don't overmix or they will become gummy.) Add more milk if necessary until you reach the texture and creaminess you prefer. Taste, and add more salt and pepper if necessary.
Transfer the potatoes to a serving dish, add a few knobs of butter on top, and garnish with some freshly chopped herbs like parsley, thyme, or chives. I don't recommend raw rosemary as it's very tough and potent.