In a bowl,using a wooden spoon, your hands or an electric mixer, combine butter and cream cheese; stir in the salt. Gradually add the flour, making a sticky dough. Sprinkle the outside of dough with flour, wrap in plastic and refrigerate for at least 1 hour and up to 24 hours. Preheat oven to 350. Working with 1/4 of the dough at a time roll it out to less than 1/4 inch thickness., using flour as necessary to prevent sticking. Cut into 2 inch squares and put a dollop of preserves in the center of each square and spread it across the diagonal, stopping short of the edges. Roll up and seal or fold into triangles. If folding on the diagonal, bring up the corners and pinch across the center. Brush with egg whites, sprinkle with sugar, place on greased or non-greased cookie sheet and bake for 10 to 15 Min. :o)