Cream together the softened unsalted butter, brown sugar, and granulated sugar for about 4 minutes until light and fluffy.
Add creamy peanut butter, egg, milk, and vanilla extract. Mix for another 2 minutes until well combined.
Stir in flour, baking soda, and salt until just combined. Chill the dough in the fridge for about 30 minutes.
Preheat your oven to 375 degrees Fahrenheit.
Roll chilled dough into 1-inch balls, roll in sugar, and place on ungreased baking sheets. Bake for 8-10 minutes.
Press a Hershey Kiss into each cookie while they are still warm.
Allow cookies to cool on the sheet for a few minutes before transferring to a wire rack.