Using a large mixing bowl, whisk together the flour, baking soda, cinnamon and salt; set aside. Using an electric mixer, cream together the butter, peanut butter, ½ cup granulated sugar and light brown sugar. Add the egg, milk and vanilla; beat until blended. Gradually add in the flour mixture with the mixer on low speed until everything is incorporated. If the dough is very soft, refrigerate for about 1 hour. If not, proceed with recipe.