Preheat the oven to 375°F. Line two baking sheets with parchment; if you don't have parchment, don't grease the baking sheets; it's not necessary., In a large bowl, beat the butter and peanut butter until well blended., Add the sugars; beat until light and fluffy., Add the egg, milk and vanilla, and beat well., Stir in the flour, soda and salt., Shape the dough into 1" balls, and roll or shake them in sugar. Place the balls on the prepared baking sheets, spacing them about 1 1/2" apart., Bake the cookies for 10 to 12 minutes, or until they appear set in the center. Remove them from the oven and immediately place one chocolate chunk or "kiss" on each cookie., Transfer the cookies to a rack to cool completely. Store at room temperature, well-wrapped, for up to a week; freeze for longer storage.