How to Make It
Place potatoes in a Dutch oven, and add cold water to cover by 2 inches. Bring to a boil over high. Reduce heat to medium-high, and simmer, uncovered, until potatoes are fork-tender, 20 to 25 minutes. Drain and let stand at room temperature until potatoes look dry and chalky, about 10 minutes. Return potatoes to Dutch oven.
While potatoes simmer, bring olive oil, rosemary, and thyme to a gentle simmer in a saucepan over low. Cook, stirring once, until mixture is fragrant and herbs have darkened, about 10 minutes. Remove from heat, and cool 10 minutes. Using a rubber spatula, press oil mixture through a fine wire-mesh strainer into a small bowl. Discard rosemary and thyme.
Mash potatoes with a potato masher until all large chunks are broken down and potatoes are fluffy. Stir in herb-infused oil, yogurt, butter, salt, and finely ground pepper until blended. Garnish with coarsely ground pepper and herb leaves.