In a medium bowl whisk together the dry ingredients. In the bowl of the mixer cream butter and sugar, about 3 mintues. Continue to beat and add in the melted chocolate, egg and vanilla.
Then alternately add the whisked flour mixture and milk until combined. Move the dough (the dough will be quite soft and stick, but it will firm up when chilled) to a piece of plastic wrap, wrap the dough and chill for at least 2-3 hours or even over night.
Divide the dough into two pieces, roll out one piece, the other piece keep refrigerate. If making the sandwich cookies then roll the dough about 1/8 inch thick. Use your favourite cookie cutter and half the cookies make a small cut out in the middle. Place cutouts on a parchment paper lined cookie sheet. Bake for approximately 10 minutes.
If making single cookies then roll the dough a little thicker about 1/4 inch thick. Place on parchment paper lined cookie sheets and bake for approximately 12 minutes.
Pre-heat oven to 350F (180C). While the oven is heating place the cut out cookies (cookie sheets in the fridge to chill again).
Let the cookies cool for a minute or two on the cookie sheet then move to a wire rack. Drizzle or dip in chocolate of fill with desired filling if making sandwich cookies, if desired.
Notes
If making sandwich cookies, place the filling on bottom of the cookie, so that they will lie flat against each other.If you wish to make chocolate chip dough then add 1/4 of mini chips. I added them to half the dough, which I gently kneaded into the chilled dough, then I made my cutouts.The filled cookies can be stored at room temperature in an airtight container for one - two days or in the fridge for up to five days. They can also be frozen in a freezer container, if filled they will last up to 1 month unfilled and they will keep for up to three months.