For the pastry: place the warm water and sugar in a 2 cup measure and sprinkle with the yeast. Stir to dissolve. Let the mixture rest for 5 to 10 minutes until bubbles form and it doubles in volume.

Combine the flour and salt in a large bowl. Add the butter. If using your hands, cut the butter into small bits with a pastry blender or your fingertips. If using a stand mixer, use the paddle attachment, but be careful not to over mix - use the lowest speed and stop when you can still see small pebble sized bits of butter in the flour.

Stir the yeast mixture down and add the egg yolks, mixing to combine. Add the yeast and egg mixture to the flour and combine until the dough comes together (it will be sticky - don't worry!). Divide the dough into four pieces. Shape into a ball and flatten with your hand to form into a 4" puck. Wrap each one in plastic wrap and refrigerate overnight.

When ready to bake, preheat the oven to 350°F.

Remove the dough from the refrigerator and let it sit out for about 15 minutes before rolling, so that it softens a bit and become easier to handle.

For the filling: Combine the walnuts with the cinnamon in a small bowl and set aside. Beat the egg whites by hand or with a hand mixer in a glass or stainless steel bowl until light and opaque. Add the sugar gradually and continue beating until stiff peaks form.

On a lightly floured surface, roll the dough out to an approximate 12" circle. (Do not worry about perfection or attempt to re-roll the dough - it usually gets worse not better and the dough will be heavier. You can always patch up the uneven parts or make some smaller and some larger). Cut each round into twelve equally sized wedges. Place ½ to 1 teaspoon of the egg whites on the wide end of each dough wedge. Sprinkle with the cinnamon and walnut mixture.

Starting from the wide edge of each dough triangle, fold the dough over the meringue filling and roll the rugelach up tightly. Place on an ungreased baking sheet with the point down. Refrigerate if not baking immediately.

Bake the rugelach until lightly browned, about 20 to 22 minutes.

Remove the rugelach from the oven and immediately toss in the powdered sugar (use your fingers if they can stand the heat or use two forks or tongs to gently toss them in the sugar). Cool on a wire rack. Enjoy them while they last!