The potato flour makes a very tender crust. These are best eaten the same day they are baked, but prepare dough one day ahead. Ideally, pearl sugar should be used in the sugar/nut mixture for sprinkling. Pearl sugar can be bought at Scandinavian shops. If you can't find it, just use crushed sugar cubes.
Sift the all-purpose flour, potato flour and white sugar together. Cut in the butter until the mixture resembles coarse crumbs. Add the egg and mix just until the dough begins to gather together. Form the dough into a ball, then flatten into a disk. Wrap in plastic wrap and refrigerate overnight.
Divide dough in half and soften at room temperature for 15 minutes.
Preheat oven to 350 degrees F (175 degrees C). Line baking sheets with parchment paper.
Knead dough briefly to soften, adding more flour it too sticky to work with. Roll 1 piece of dough out between sheets of floured wax paper to a thickness of 1/8 inch. Cut out rounds that are 2 1/2 inches across using a fluted cookie cutter or tart mold. Transfer cookies to the prepared baking sheet. Repeat with remaining dough.
Place 1/4 teaspoon preserves in center of each cookie. Gently fold half of cookie over preserves, covering 2/3 of bottom half of cookie (a little bit of the bottom layer of the cookie should be showing). Combine pearl sugar and nuts. Brush cookies lightly with beaten egg white. Sprinkle generously with nut mixture.
Bake at 350 degrees F (175 degrees C) for 10 minutes or until a pale golden color. Cool on baking sheets for 10 minutes then transfer to rack and cool completely.