Sift the flour, cocoa powder and salt into a large bowl.
Beat the soft butter and powdered sugar in another large bowl for 2 minutes, until creamy.
Add the dry ingredients to the butter. Mix on low speed just until the dough comes together.
Shape the dough into a log about 2 inches wide and 8 inches long. Wrap it tight in parchment paper.
Chill the log for at least 1 hour.
Line two baking sheets with parchment paper. Slice the log into rounds about 1/3 inch thick and place them 1 inch apart.
Heat the oven to 320°F (160°C). Chill the cut cookies while the oven heats. Bake for 12 to 15 minutes, until the centers are soft but not sticky.
Cool on the sheet for 5 minutes, then move to a wire rack to cool completely.