Preheat the oven to 375°F and line baking sheets with parchment paper. In a large bowl or the bowl of a stand mixer fitted with the paddle, blend the peanut butter, unsalted butter, both sugars, and salt. Beat on medium speed until smooth and fluffy. Add the egg and vanilla extract, mixing thoroughly and scraping down the sides as needed. Incorporate the flour and baking soda, beating on low just until the dough comes together. Shape the dough into 1-inch balls and roll each in additional granulated sugar if desired. Arrange on the prepared baking sheets, spacing them 2 inches apart. Bake for 8–10 minutes, or until the cookies appear puffed and cracked with lightly browned bottoms. Immediately press a chocolate kiss into the center of each cookie while warm; the tops will crack slightly. Let the cookies cool entirely on the baking sheet until the chocolate sets. Store in an airtight container for up to a week.