Put the pasta flour into a mixing bowl and make a well in the middle.
Pour the egg yolks into the well and then using a fork, slowly whisk the egg yolks and then the pasta flour together.
Turn the pasta dough out onto a clean work surface dusted in pasta flour and knead for around 10 minutes until it forms a smooth and springy ball that is dry to the touch. Wrap in clingfilm and rest for at least 30 minutes in the fridge.
Once rested, take the pasta dough and cut into 4 pieces. Roll out each piece one at a time into a rectangle 15cm wide and 25cm long (6X10 inches).
If using a pasta machine, feed the piece of pasta dough through on the widest setting twice, before gradually lowering the setting until you reach a thickness of 6mm. Now use the tagliatelle attachment to cut into thin, flat noodles.
If using a roller, fold the rectangle of pasta inwards on itself like an envelope, turn 45 degrees and then roll out again. Repeat this step a few times rolling it thinner each time. Finally, dust with semolina flour before folding it up and cutting it into thin, flat noodles with a knife.
Move the tagliatelle to a baking tray dusted with semolina flour, adding more over the top to prevent it from sticking together. Allow the tagliatelle to rest for a further 30 minutes.
Heat butter in a large frying pan and then fry the sage leaves until crispy, then set to one side.