Preparation - Mix the flour, cocoa, and salt together. In a separate mixing bowl, combine the butter, sugar, and vanilla. Beat until well blended. Add the flour mixture and mix until well blended. Wrap the cookie dough in waxed paper or plastic wrap and refrigerate for at least 4 hours before using. On a lightly floured surface, roll a portion of the dough to about 1/4" thick. Cut with cookie cutters. Place the unbaked cookies on non-stick baking sheets, about 1" apart. Bake at 350º for about 8 minutes. Remove from the oven and allow the cookies to cool on the pan for about a minute, then transfer them to a wire rack to cool completely. Re-roll and use the trimmings. Repeat with the remaining cookie dough. Decorate the cooled cookies as desired.