Place potatoes into a 5-quart stockpot and cover with 3 quarts of cold water. Add kosher salt and bring to a simmer over high heat. Reduce heat to medium-low and simmer gently until potatoes are just tender, 15 to 20 minutes. Remove from heat and drain well. Let potatoes cool just until they're safe to handle.

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

Combine butter, sliced 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 metal baking sheets with silicone baking mats (such as Silpat®) and generously brush on some of the infused melted butter.

Remove potatoes from the fridge and gently smash each between two pieces of plastic 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 up into small pieces. Transfer onto a sheet pan, being careful not to overlap potatoes. Very generously drizzle and brush most of the melted butter on top.

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

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

Carefully transfer potatoes onto a serving platter and scatter over the golden brown slices of garlic. Crumble the toasted herbs on top if desired. Serve immediately.