Stir flour, cocoa and salt together. Set aside. In large mixing bowl, cream butter till fluffy. Add flour mixture, cream and vanilla. Beat at low speed until well-mixed. Cover bowl with plastic wrap and refrigerate for 30 minutes. Grease a cookie sheet. On lightly floured surface, using a rolling pin, roll out half of dough to 1/8" thickness. Using round cookie cutter, slightly larger than mint patties, cut out 24 cookies. Put the 1/4 cup sugar in a small bowl. Dip the cookies in the sugar (coat both sides) and place on cookie sheet. Place mint pattie on each cookie. Roll out remaining dough and coat 24 more cookies with sugar. Place these cookies on top of mints. Press fork firmly around each cookie to seal. Bake at 375 for 10 minutes. Cool 1 minute, then remove from cookie sheets to rack, to cook completely.