In a large bowl, cream butter and 3/4 cup sugar until light and fluffy. Beat in egg and vanilla. Combine the flour, baking powder and salt; gradually add to creamed mixture and mix well. Stir in mincemeat. Cover and refrigerate for 2 hours. , Roll dough into 1-in. balls; dip into egg white and remaining sugar. Place sugar side up 2 in. apart on greased baking sheets. Bake at 375° for 10-12 minutes or until set. Remove to wire racks.