Stir the sifted flour, baking soda, cream of tartar, and salt together in a bowl.
Beat the margarine, butter and sugar together with an electric mixer in a large bowl until smooth. Add one egg, and continue beating until completely incorporated; add the remaining egg along with the vanilla extract, and beat on low speed until incorporated. Mix the flour mixture into the butter mixture until just incorporated. Gently mix the pecans into the dough with your hands.
Roll the dough into 1-inch balls; arrange onto baking sheets about 2 inches apart.
Bake in the preheated oven until the edges are golden-brown, 8 to 9 minutes. Allow the cookies to cool on the baking sheet for 8 minutes. Place confectioners' sugar into a bowl, and lightly dip both sides of each warm cookie into the sugar. Tap of excess, and allow cookies to finish cooling.
