Place potatoes into a 5-quart stockpot and cover with 3 quarts cold water. Add kosher salt; bring to a simmer over high heat. Reduce heat to medium-low and simmer until potatoes just tender, 15 to 20 minutes; drain well. Let potatoes cool just until cool enough to handle.

Transfer potatoes to a baking sheet; continue cooling to room temperature. Make 4 or 5 shallow cuts down sides of each potato, about every inch or so, to ensure skin splits evenly when smashed. Refrigerate until completely chilled and ready to smash; 8 hours to overnight.

Heat butter, garlic, rosemary, and thyme in a small pan over medium heat; cook, stirring occasionally, until butter melts and begins to bubble, and garlic softens and starts to turn translucent, 3 to 5 minutes. Remove from heat.

Preheat the oven to 450 degrees F (232 degrees C). Line 2 baking sheets with silicone baking mats (such as Silpat); generously brush on some infused butter.

Gently smash each potato between two pieces of plastic wrap using a flat, heavy object until 1/2- to 3/4-inch thick. Season generously with salt, pepper, and cayenne on both sides, being careful not to break potatoes into small pieces; transfer to prepared baking sheets, being careful not to overlap potatoes. Very generously drizzle and brush most of infused butter on top.

Bake in the preheated oven until well browned and crunchy around edges, 35 to 45 minutes.

Meanwhile, place the pan with remaining infused butter back over medium-low heat. Cook and stir until garlic starts to turn a very light golden brown, 3 to 4 minutes. Remove from heat and reserve.

Carefully transfer potatoes to a serving platter; scatter golden brown garlic slices and crumble toasted herbs on top, if desired. Serve immediately.