Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.
Gently pull the skin off the chicken breasts (reserve the breast meat for another use), pat dry and cut in thirds lengthwise. Place them skin-side up in a single layer on the prepared baking sheet.
Mix the salt, pepper, thyme and rosemary together in a small bowl, crushing the spices with a spoon. Sprinkle the chicken skins with some of the spice mix; it makes more than you need, but you can keep it for another day. Cover the skins with a layer of parchment and top with a baking sheet to weigh it down, making sure the top sheet presses down on the skins.
Bake until the skins are browned and crisp, 35 to 40 minutes. Remove the top baking sheet and parchment and transfer the skins to a plate lined with paper towel to cool and crisp up.