Using knives, cut together flour and butter until butter is small marble size. Make a well, and add sweetener, egg yolk, water and lemon rind. Quickly combine all ingredients. Place dough on floured surface and knead lightly together for 5 minutes. Roll dough into a ball, and chill in the refrigerator for half hour. Roll dough to 1 cm thickness, and cut into 10 cm squares. Put a tablespoonful of jam in the center of each square, fold into a triangle and seal the edges. Place on oiled cookie sheets and bake in preheated 400 degree oven for 10 minutes. Tarts will be golden brown when done. Serve warm or cooled. Makes dozen tarts