Preheat oven to 425°. In a large pot, cover potatoes with water and add a generous pinch of salt. Bring water to a boil and simmer until potatoes are tender, about 15 minutes. Drain and let sit until cool enough to handle.
On a large rimmed baking sheet, toss potatoes with oil, season with kosher salt. Using the flat bottom of a glass or mason jar, press down on potatoes to smash flat.
Bake until bottoms of potatoes are beginning to crisp and golden, about 20 minutes.
While potatoes are cooking, melt the butter in a small pan over low heat. When butter is melted, stir in the garlic and herbs. Remove from heat and set aside.
Spoon the melted butter, garlic, rosemary and thyme mixture over the potatoes, brush to evenly coat them.
Return potatoes to oven for 5 minutes. Serve immediately.