Preheat the oven to 375°F and line two large baking sheets with parchment paper.

In a medium bowl, whisk together the flour, baking soda, and salt.

In the bowl of a stand mixer, beat the butter, brown sugar, and 1/4 cup granulated sugar until light and fluffy. Add the peanut butter, egg, milk, and vanilla and mix until creamy. Gradually add the dry ingredients and mix on low speed until just combined.

Scoop heaping tablespoons of the dough, then use your hands to roll it into balls. Roll the dough balls in the remaining 1/2 cup granulated sugar and arrange them on the prepared baking sheets, leaving at least 1 inch between balls.

Bake, one sheet at a time, for 8 minutes, or until fine cracks start to form on the tops of the balls. Remove from the oven and press a chocolate kiss into the top of each cookie until the edges start to crack. Use a spatula to immediately transfer the cookies to a wire rack to cool completely.