Step 1

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.

Step 2

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.

Step 3

Preheat oven to 375 degrees F (190 degrees C). Lightly grease baking sheets.

Step 4

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.

Step 5

Bake in preheated oven until edges are lightly browned, 12 to 15 minutes. Cool on racks, and store in an airtight tin.