Position oven racks in the upper and lower thirds of the oven and preheat to 350°F. Line two large baking sheets with parchment paper. Brush the egg over all sides of the bars, avoiding the ends. Spread the decorative sugar on a sheet of parchment or wax paper, then press the bars into the sugar to coat them evenly. Cut each bar crosswise into 1/4-inch slices, rotating the bar after each cut to help keep the slices square. If the dough becomes too soft, briefly freeze the bars until firm enough to slice. Arrange the cookies about 1/2 inch apart on the prepared baking sheets. Bake, switching the sheets between racks halfway through, until the edges are pale golden, 15–18 minutes. Transfer the cookies to racks to cool completely.