Beat the butter, sugar, egg, and salt together in a mixing bowl until light and fluffy. Stir in the flour and baking soda, and mix just until blended. Mix in the hazelnuts.
Turn the dough out onto a lightly floured surface and shape into 2 1/2 inch log. Wrap snugly in wax paper, and refrigerate overnight.
Preheat oven to 375degrees F (190 degrees C). Lightly grease baking sheets.
Unwrap, and cut the log into 1/8 inch thick slices. Place on prepared baking sheets 2 inches apart. Brush tops with beaten egg and, if desired, decorate with sprinkles.
Bake in preheated oven until edges are lightly browned, 12 to 15 minutes. Cool on racks, and store in an airtight tin.
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