In the bowl of a stand mixer fitted with the paddle attachment*, mix the granulated sugar, light brown sugar, and softened butter. Add the peanut butter, sourdough discard, egg and vanilla extract, mixing on medium speed until smooth and combined.
Add the flour, baking soda and salt and mix until no dry spots remain.
Use a cookie scoop to divide the dough into 1 Tablespoonfuls, then roll each into a ball between your palms. Roll each dough ball in granulated sugar (or sprinkles, if using) to coat, then place on a lightly greased baking sheet, about 2 inches apart.
Bake for 8-10 minutes until slightly golden. Remove from the oven, and let sit for 2-3 minutes, then press a Hershey's Kiss in the center of each cookie while the cookie is still warm. Transfer the cookies to a cooling rack to cool fully.
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Notes
*You do not need to use a stand mixer to make this recipe. You can also use a large mixing bowl and a hand-held mixer.