Cream the butter and sugar together until fluffy. Blend in the beaten eggs and then the flour, little by little. Add the soda and vanilla extract and blend well. Chill the dough in a covered bowl until firm. Add more sifted flour, but not one speck more than is necessary to reach a consistency that can be rolled out. On a floured surface, roll about ⅛ inch thick and cut in rounds about 3 inches across. In the center of half the rounds place a thin chocolate mint. Place another round on top and press the edges together with a fork. Press a pecan half on each cookie. Bake on buttered cookie sheets in a hot (425 degrees F) oven until lightly browned-about 12 minutes.