In a large bowl using an electric mixer or in the bowl of a stand mixer fitted with the paddle attachment, beat the softened unsalted butter, creamy peanut butter, 1/3 cup granulated sugar, and 1/3 cup packed light brown sugar together on medium speed for 2 minutes until smooth and pale beige.

Add the egg, whole milk, and pure vanilla extract to the butter mixture, beating on medium speed for 1 minute until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, baking soda, and fine salt.

Add the flour mixture to the wet ingredients and mix on low speed just until a soft dough forms and no dry streaks remain.

Cover the bowl tightly with plastic wrap and refrigerate the dough for 30 minutes to firm up the fats.

Preheat your oven to 375°F (190°C) and line two large baking sheets with parchment paper.

Place the remaining 1/3 cup granulated sugar in a small, shallow bowl.

Scoop tablespoon-sized portions of chilled dough (about 22 grams each) and roll them between your palms into smooth 1-inch balls.

Roll each dough ball in the granulated sugar until completely coated, then place them on the lined baking sheets spaced 2 inches apart.

Bake one sheet at a time at 375°F (190°C) for 8 to 10 minutes, until the cookie tops are puffed and edges are set with fine surface cracks.

Remove the baking sheet from the oven and immediately press one unwrapped milk chocolate kiss firmly into the center of each hot cookie within 60 seconds.

Allow the cookies to rest on the hot baking sheet for 3 minutes, then carefully transfer them to a wire cooling rack to cool completely for 2 hours until the chocolate resets.