Preheat oven to 350F. Line two cookie sheets with parchment paper. Prepare a rolling area with two additional sheets of parchment paper for that, and have your cookie cutter(s) handy.
Place butter in a large mixing bowl and whip it with a mixer until it's creamy. Add sweetener, vanilla, and salt and mix once again to combine. Add in flour, cocoa powder, and milk and use a wooden spoon to mix. Then get in there with your hands and mix everything together by working the dough until you can shape it into a ball {note: as depending on the flour mix you use there may be a slight variance, know that the consistency of the dough should not be sticky but should press together when pinched -- be sure to knead it really well first for some time -- if it's a little sticky, add a little more flour (try 1-2 tbsp); if it's a little dry add a little more sweetener or milk (try 1 tbsp)}. Shape the dough into 2 balls and then flatten each into a disk.
Roll out one of the dough balls between two sheets of parchment paper to 1/4" thickness. Use a cookie cutter to cut out the cookies. Carefully transfer to a prepared cookie sheets, spacing them ½" apart {they won't spread as they bake}. Gather up any dough scraps and repeat until all dough is used up. Repeat the process with the second dough ball.
Bake in a pre-heated oven for approximately 11-13 minutes, until the edges just begin to darken. Remove from oven and place on a cooling rack. {Note: cookies will harden a little within minutes of cooling, so don't overbake}. Allow the cookies to cool for 10 minutes and enjoy!
Notes
See optional frosting or icing ideas at the top of this post.