1. Preheat your oven to 375°F or 190°C.

2. In a large bowl, combine the beef and pork mixture, egg yolks, salt, and black pepper.

3. Mix well with your hands or a wooden spoon until just combined, being careful not to overmix.

4. Use your hands to shape the mixture into small meatballs, about 1 1/2 inches or 3.8 cm in diameter.

5. Place the meatballs on a baking sheet lined with parchment paper, leaving about 1 inch or 2.5 cm of space between each meatball.

6. Dot the top of each meatball with butter or margarine.

7. Bake in the preheated oven for 12-15 minutes, or until cooked through.

8. While the meatballs are baking, prepare the cranberry or red currant jam, which is the English equivalent of lingonberry jam.

9. In a medium saucepan, combine the thawed cranberries or red currants and granulated sugar.

10. Bring to a boil over medium-high heat, then reduce the heat to medium-low and simmer for 10-15 minutes, or until the jam has thickened slightly.

11. Serve the meatballs hot with boiled potatoes and a spoonful of the cranberry or red currant jam on the side.