Preheat oven to 325 degrees F with racks in upper and lower thirds. Line 2 large baking sheets with parchment paper.

Whisk together flour and baking soda.

Beat butter with an electric mixer until fluffy (if using a stand mixer, use paddle attachment), then add sugar and flour mixture, mixing until just combined. Divide dough in half.

Roll each piece of dough between large sheets of plastic wrap into a 15-by-10-inch rectangle (about 1/8 inch thick). Chill on a tray until firm, about 30 minutes. Cut into 2- by 1-1/2-inch rectangles and arrange 1 inch apart on 2 large baking sheets. (If dough becomes too soft, chill or freeze until firm enough to handle.)

Brush tops of cookies lightly with beaten egg, then sprinkle with sanding sugar. Bake cookies,

1. 5Bake cookies, 2 sheets at a time, switching positions of pans halfway through baking, until cookies are very pale golden, about 15 minutes. Cool on sheets 5 minutes, then carefully slide cookies (on parchment) to racks to cool completely. Make more cookies with remaining dough, baking on cooled, freshly lined baking sheets. Re roll scraps once. Can be made 1 day ahead.