In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
Using a hand mixer or stand mixer, beat the butter and peanut butter together until creamy (about 1–2 minutes).
Add the brown sugar and ¼ cup of granulated sugar. Mix on medium speed until smooth and creamy, scraping down the sides of the bowl as needed.
Add the egg and beat on high until fully incorporated. Mix in the vanilla extract and milk until combined.
Gradually add the dry ingredients to the wet mixture. Mix on low speed just until the dough comes together.
Cover the dough and chill for 1 hour. If chilling longer, let the dough sit at room temperature for about 15 minutes until it softens enough to scoop.
Preheat the oven to 350°F (177°C). Line two baking sheets with parchment paper or silicone baking mats. Place the remaining ½ cup of granulated sugar in a small bowl.
Scoop about 1 tablespoon of dough and roll it into a ball using your hands. Roll the dough ball in sugar and place it on the prepared baking sheet, spacing the cookies about 2 inches apart. Repeat with the remaining dough.
Bake for 8–10 minutes, or until the tops show small cracks. The cookies should not brown. Remove from the oven and cool on the baking sheet for 5 minutes.
Gently press a Hershey’s Kiss into the center of each cookie. Allow the cookies to sit on the baking sheet for another 5–10 minutes before carefully transferring them to a wire rack to cool completely.