Preheat oven to 350F. In a large mixing bowl, cream together butter and sugar. Add in flour, baking powder and salt at a low speed, then add in vanilla and milk. The dough should come together into a soft ball of dough.
Take a tablespoon full of dough, roll it into a ball and then slightly flatten it into a disc about 1/4-inch thick. Place on a baking tray and repeat with remaining dough. NOTE: Cookies do not spread so you can fit quite a few on the tray.
Bake cookies for 11-13 minutes, until bottoms and the edges are lightly browned and cookies are set. Immediately after removing cookies from the oven, use your thumb or a small spoon to make a depression in the center of each cookie. You have to do this immediately or the cookies will cool and when you try to press into them, the cookies will crack at the edges. That is bad! I even waited too long and couldn’t get the depression deep enough to add as much peanut butter filling as I would like!
Let cookies cool for a few minutes on the baking sheet then transfer to a wire rack to cool completely.
Time to make the peanut butter filling. In a small bowl, whisk together peanut butter, powdered (confectioners) sugar, salt and vanilla. When the mixture is blended, heat it in the microwave for about 30 seconds (watching it closely) until it is very, very soft – almost liquid-y. Transfer the hot liquid (be careful!) it to a pastry bag (or plastic bag with the tip cut off – this is what I did!) and squeeze a generous dome of the filling into each cookie’s “thumbprint”.
Put the cookies in the refrigerator and chill cookies with filling for 20-30 minutes, or until the peanut butter is firm.
Melt the chocolate either in the microwave on on a double boiler. Once melted, if you place the chocolate in a pan above another pan of hot, but not boiling water (aka double boiler-type situation!) to keep it fluid while you work, it’s much easier to work with.
Remove the chilled cookies from the fridge and dip one at a time into chocolate, let excess drip off, and place on a sheet of parchment paper to let the cookies set. I placed the cookies directly on a cooling rack over waxed paper to let the chocolate drip off a bit and it worked out great but it did leave lines on the bottom of the cookies. You can also chill the cookies again to accelerate the process.
Once the cookies are set- you can enjoy the deliciousness that is Homemade Tagalong Cookies! NOTE: Next time I plan to make a deeper impression in the shortbread cookie and add more peanut butter filling. Makes 3 dozen cookies.