Gather the ingredients.

Beat butter, sugar, egg, and salt together in a mixing bowl until light and fluffy. Stir in flour and baking soda, and mix just until blended. Mix in hazelnuts.

Turn dough out onto a lightly floured surface and shape into 2 1/2 inch log. Wrap snugly in wax paper, and refrigerate overnight.

Preheat the oven to 375 degrees F (190 degrees C). Lightly grease baking sheets. Unwrap, and cut the log into 1/8 inch thick slices.

Place 2 inches apart on the prepared baking sheets. Brush tops with beaten egg and decorate with sprinkles, if desired.

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