5 egg yolks
2 tablespoons grapeseed or any mild-flavored oil
In the bowl of a food processor fitted with a metal blade, combine the flour, salt, and baking powder and process in 3 or 4 (1-second) pulses to mix. Add the eggs, egg yolks, and oil and process for about 50 seconds.
Stop the processor and feel the dough with your hand. It should feel supple and soft, but not tacky. This is a smooth dough; it should be pliable but not sticky. If it is not completely together or not yet soft, smooth, and supple, process for another 10 to 15 seconds (you are actually kneading the dough right in the food processor). Transfer the dough to a bowl, cover with plastic wrap, and let stand at room temperature for about 2 hours, allowing the gluten in the flour to relax.
When the dough has rested, cut it into 4 equal parts. Place on a work surface and roll one piece into a 12- by 16-inch rectangle that is less than 1/16th of an inch thick or as thin as necessary to measure that size rectangle. Cut lengthwise into 4 (3- by 16-inch) strips.
Stack the sheets on top of each other without pressing them down. Then, with a long, very sharp knife, cut the strips lengthwise into 1/4-inch-wide noodles. (These expand a bit as they cook, but you can play with the length and width if you like.) Repeat with the remaining dough.
Cook in salted boiling water for 2 minutes and drain in a fine-mesh sieve. Add to any hot, brothy soup, cook for 1 to 2 minutes, and serve, or top with any stew or a light, fresh tomato sauce.