Beat together the butter and powdered sugar until combined.
Sift in the cocoa powder and almost all of the flour (except for about 2 tablespoons). Add in the salt.
Stir the mixture a few times by hand. Then beat in the dry ingredients starting with the mixer on a low speed.
When about 90-95% of the dry ingredients are incorporated, dump the dough on a lightly floured surface (including the flour and crumbs at the bottom of the bowl) and knead a few times to incorporate the remaining dry ingredients/crumbs.
If the dough feels sticky and is sticking to your hand, sprinkle the remaining flour overtop and knead in the remaining 1-2 tablespoons of flour. Only knead the dough a few times. Do NOT continue kneading or mixing after the flour is incorporated.
Form the dough into a tight log about 1.5 inches (3.5-5 cm) in diameter. Wrap in parchment paper or wax paper and twist the ends.
Refrigerate for at least 1 hour (I prefer at least 2) or up to 1 week.
When ready to bake, preheat the oven to 325F (160C). Line cookie sheets with parchment paper or silicone baking mats.
Unwrap the log and slice the dough using a sharp knife. You want each cookie to be about ¼ inch (⅓ - ½ cm) thick. Place cookies 1 inch apart on the lined cookie sheets.
Bake 1 sheet at a time in the preheated oven for approximately 14-16 minutes, or until the tops look set. Keep the remaining unbaked cookies in the fridge so that the dough doesn't get too warm.
Cool the cookies on the tray for at least 10 minutes, then transfer to a wire rack to continue cooling
To melt the chocolate, chop it into very fine pieces (if using a block/bar). Place chocolate in a heatproof bowl.
Microwave on medium power (not high power, which is often standard) for 45-second intervals. Stir between each interval until smooth.
Dip half of each cookie in chocolate. Alternatively, transfer the chocolate to a piping bag with a small writing tip (or a freezer bag with the corner cut off) and drizzle on top of each cookie. Optionally, decorate with sprinkles. Wait for the chocolate to harden before stacking the cookies.