How to Make It
In a large bowl, combine the flour and salt. In another bowl, beat the egg yolks with the whole eggs and cream. Stir the egg mixture into the flour until a dough forms. Knead and shape the dough into a smooth ball. Flatten the dough into a disk, wrap in plastic wrap and let rest at room temperature for 1 hour.
Cut the dough into quarters. On a lightly floured work surface, roll out one-quarter of the dough 1/8 inch thick. Cut the dough into 3-inch-wide strips. Cut each strip crosswise into 1/8-inch-wide noodles and transfer to a baking sheet. Repeat with the remaining dough.
Bring a large pot of salted water to a boil. Add the noodles and cook until tender, about 4 minutes; drain. Add the butter to the pot and melt over low heat. Add the noodles and celery leaves and toss well. Season with salt and pepper and serve at once.
Make Ahead: The spaetzle dough can be refrigerated overnight. Let return to room temperature before rolling out.