To make the filling, heat the butter in a frying pan. Cook onion, garlic and prosciutto over moderately low heat for 5-6 minutes without browning. Add the pork and veal, increase the heat and lightly brown, breaking up the lumps. Add the oregano and paprika, season well and stir in the tomato paste and chicken stock.
Cover pan and cook for 50 minutes. Uncover, increase the heat and cook another 10 minutes until the filling is quite dry. Cool, then chop to get rid of any lumps. Stir in the egg yolks.
To make the ravioli, divide the rolled out pasta dough into four sheets: two 30 x 20 cm (12 x 8 inch) sheets and two slightly larger. Dust work surface with semolina and lay out one of the smaller sheets (keep the rest of the pasta covered with a damp tea towel to prevent it drying out). Lightly score the pasta sheet into 24 squares. Place a scant teaspoon of filling in the centre of each square and flatten it slightly with the back of the spoon.
Brush beaten egg along the score lines around the filling. Take one of the larger pasta sheets and cover the first, starting at one end. Match edges and press the top sheet onto the beaten egg as you go. Avoid stretching the top sheet, rather let it settle into place. Run your finger firmly around the filling and along the cutting lines to seal well. Use a sharp knife to cut into 24 ravioli squares.
If you are not using the ravioli immediately, place them on a baking paper dusted with cornmeal and cover with tea towel. They can be left for 1-2 hours before cooking. Do not refrigerate or they will become damp.
Cook ravioli, in small batches, in a large saucepan of boiling salted water until al dente. Remove and drain with a slotted spoon. Serve on their own or with melted butter, olive oil or grated parmesan.