Line a large rimmed baking sheet with wax paper. In a large bowl, blend the ricotta with the flour, Parmesan, egg yolks, 3/4 teaspoon of salt and 1/4 teaspoon of pepper. Spoon the gnocchi dough into a large, sturdy resealable plastic bag. Cut off a 1/2-inch corner from the bag. Pipe the dough onto the prepared baking sheet in five 1-foot-long strips about 3/4 inch wide. Cover with plastic wrap and freeze until firm, about 3 hours.