FOR THE DOUGH: Dissolve the yeast and sugar in the water and set aside for 5 minutes.
Place the flour in a large bowl or the bowl of a stand mixer. Add the butter and rub it in with your fingertips or mix at medium speed, to a coarse meal. Add the eggs and salt, then add the yeast mixture. Use a wooden spoon or a mixer fitted with the dough hook to mix together to a soft dough. If the dough is very sticky, knead in a little extra flour.
Place the dough in a heavy plastic bag and tie tightly, leaving room in the bag for the dough to expand. Place in a second bag and tie tightly. Fill a large bowl or pot three quarters full of lukewarm water and place the dough in the water. Let stand until the dough rises to the surface, about 1 1/4 hours.
FOR THE FILLING: Meanwhile, prepare the filling: Place the oats in a food processor and process to a slightly finer texture. Add the hazelnuts and finely chop.
Beat the egg whites in a large bowl until stiff, but not dry. Fold in the oat and hazelnut mixture, then add the remaining ingredients and fold in to make a coarse paste. Set aside.
Place two racks just above and below the center of the oven and preheat the oven to 375°F Line two baking sheets with parchment, or lightly grease them.
TO FILL COOKIES: Turn the dough out onto a well floured surface and knead briefly. Cut into 6 equal pieces. Sprinkle about 2 Tablespoons sugar onto an un-floured part of the work surface. Work with one piece of dough at a time, leaving the others loosely covered with plastic: Flatten the dough on the sugared surface with the palm of your hand, then roll out to a circle about 6 inches across and just less than 1/4 inch thick. Cut into 8 wedges. Place 1 1/2 teaspoons filling across the WIDEST part of each wedge, then roll each up toward the point.
Place on a prepared baking sheet point side down, leaving an inch between them. Repeat to shape the remaining cookies, sprinkling a little sugar (1 or 2 Tablespoons) on your work surface each time before rolling out the dough. Let the cookies rise, loosely covered, for 15 minutes.
Brush the cookies with the egg wash and sprinkle with a little sugar. Bake for 12 minutes, or until lightly golden. Let cool on fine mesh racks before serving. Makes 4 dozen hazelnut filled rolled up cookies.
NOTE: Hazelnuts known as noisettes in French, are sometimes called filberts in the United States. Shelled hazelnuts still have their papery thin brown skins. The easiest way to remove the skins is to lightly roast the nuts and then rub them between your hands or in a towel. Don’t worry if some skin remains on them.