In a bowl, mix the flour, cocoa powder and salt with a whisk or fork until well blended.
In the bowl of a stand mixer, beat the butter and sugar until smooth and fluffy. Beat in the yolk and scrape down the bowl to make sure everything is incorporated.
Stop the mixer and put in all the flour. Mix on low until all is incorporated. It will begin to form up into clumps.
Put a piece of plastic wrap on the counter, scoop out the dough, and form it into a disk on the plastic. Wrap it and refrigerate it for about an hour.
When the dough is chilled, put it between two sheets of parchment paper and roll it out to about ½-inch to ¼-inch thick.
Heat the oven to 325 F and line two cookie sheets with parchment. Using the biscuit cutter, stamp out the cookies and place them on the pans.
Reroll and keep cutting until you have used up all the dough. Bake the cookies for about 20 minutes. They will have puffed just a bit and the tops will be set.
Rotate the sheets halfway through the cooking time. When done, put the pans on wire racks to cool completely.
As the cookies approach cool, in a microwave safe bowl, melt the chocolate chips in the microwave and stir the chocolate until smooth.
Dip half of each cookie into the melted chocolate and place it on the parchment-covered pan to dry. Plate the cookies when the chocolate is dry.
