In a medium mixing bowl, whisk all-purpose flour, baking soda, and salt.
In a large bowl, beat softened unsalted butter, peanut butter, castor or granulated cane sugar, and light brown sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed to ensure the butter and sugar is evenly combined.
Next, add the egg, milk, and vanilla extract, and beat until just incorporated.
Add the dry ingredients in two batches, and beat on low speed just until the dough comes together. Use a spatula to fold in any leftover flour streaks to avoid overworking the dough. Cover the bowl and chill for 30 minutes.
Preheat the oven to 375°F (190°C). Line baking trays with parchment paper.
Place about 1/4 cup of granulated sugar in a shallow bowl. Shape the cookie dough into 1-inch balls (use a tablespoon or cookie scoop for uniform-sized cookies), then roll each ball in sugar to coat completely.
Place on the prepared baking tray 2 inches apart. Bake at 375°F for 8 to 10 minutes or until the centers are just set and edges turn a light golden. I baked for exactly 8 minutes.
Immediately press a Hershey's kiss gently into the center of each cookie. A little cracking is absolutely fine! Pressing too hard can cause the cookies to crack too much or spread too much.
Allow cookies to set on the baking tray for 5 minutes. Transfer to a cooling rack and allow to cool down completely!