Sift together flour, baking soda and salt; set aside.
Using an electric mixer, cream together butter, peanut butter, ½ cup granulated sugar and light brown sugar.
Add egg, milk and vanilla; beat until well blended.
Gradually add flour mixture, mixing thoroughly. If the dough is very soft, refrigerate for about 1 hour.
Heat oven to 375 F. Spray, oil or line a cookie sheet with nonstick liner and set aside.
Roll dough into 1-inch balls. (For exactly 5 dozen cookies, divide the dough into 5 pieces, and shape each piece into 12 balls.)
Roll cookies in sugar and place 2 inches apart on cookie sheet. Bake until very light brown and puffed, 6 to 8 minutes.
Remove sheet from oven and lightly press a chocolate into the center of each cookie, allowing it to crack slightly. Return to oven until light golden brown, 2 to 3 minutes. Remove from oven, cool completely and store in an airtight container.