Whisk together the flour, cocoa powder, salt, and confectioners' sugar (see "tips," below)., Work the butter into the flour mixture, using a pastry blender or fork, your fingers, or a stand mixer. Don't mix the butter in completely; leave some of it in pea-sized lumps., Add 4 tablespoons of the water, then enough additional water to just bring the dough together; it should be cohesive, but not sticky., Shape the dough into a disk, and chill until you're ready to use it.