Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large mixing bowl, combine softened butter and peanut butter. Cream together until smooth and fluffy.
Step 3: Gradually add 1 cup of granulated sugar and light brown sugar to the mixture. Mix until well combined.
Step 4: Add the egg, milk, and vanilla extract. Beat until everything is well incorporated.
Step 5: In a separate bowl, whisk together flour, baking soda, and salt. Gradually add this mixture to the wet ingredients, mixing until just combined.
Step 6: Scoop out portions of dough and roll them into balls. Roll each ball in the remaining granulated sugar.
Step 7: Place the dough balls onto prepared baking sheets, spacing them about 2 inches apart. Bake for 8-10 minutes.
Step 8: Press a Hershey's Kiss into the center of each cookie as soon as you remove them from the oven.
Step 9: Allow to cool on the baking sheet before transferring to a wire rack to cool completely.