Heat a large cast iron pan over medium heat, add the butter and allow to melt. Season the pork all over with salt and pepper then add to the pan and brown the meat on all sides, turning occasionally, about 4 minutes total. Remove to a plate and let cool.
In the bowl of a food processor fitted with the blade attachment, add the cooked pork loin and prosciutto, pulsing until the meat is finely chopped but not puréed. Add to a medium mixing bowl and stir together with the ricotta, egg yolk and parmesan.
Roll out the pasta dough into a thin sheet using a pasta maker (I use the KitchenAid pasta roller attachment set to setting 7). Transfer the dough to a work surface and divide it into two equal balls. Wrap each in plastic wrap and flatten each into a disc. Let stand at room temperature for at least 30 minutes and up to 2 hours.
Cut the sheet into 2” x 2” squares then add ½ teaspoon of filling into the center of each square. Bring the opposite corners of the square together to form a triangle and use your fingers to seal the sides shut, making sure to remove any air pockets.
Bring the two bottom corners of the triangle together so they are slightly overlapping at the front of the tortellini and press together to seal. Place the tortellini on a lightly floured baking sheet and cover with plastic wrap as you work so they don’t dry out.
Repeat with the remaining sheets of pasta and filling.