Beat butter on medium speed, until creamy, add sugar and mix well for about one minute. (The mixture should be smooth and velvety, not fluffy and airy)
Reduce mixer speed to low and add in 2 egg yolks until well mixed
Add all the flour and pulse the mixer so that it just becomes moist. You want to work the dough as little as possible. What you're aiming for is a soft, moist, clumpy (rather than smooth) dough. Pinch it, and it will feel a little like Play-Doh.
Remove dough and divide in half. Roll into two logs and refrigerate for at least 3 hours.
Whisk remaining egg. Remove logs of dough and paint the eggs on the logs. Sprinkle with decorative sugar.
Slice cookies into 1/3 inch lengths.
Bake at 350 degrees for 17-19 minutes. When properly baked, the cookies will be light brown on the bottom, lightly golden around the edges and pale on top; they may feel tender when you touch the top gently, and that's fine. Remove from the oven and let the cookies rest a minute or two before carefully lifting them onto a rack with a wide metal spatula to cool to room temperature.