Step 1
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.
Step 2
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.
Step 3
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.
Step 4
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.
Step 5
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.
Step 6
Bake potatoes in the preheated oven until well browned and crunchy around the edges 35 to 45 minutes.
Step 7
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.
Step 8
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.