In a medium saucepan, heat the olive oil over medium heat.
Add the chopped onion, garlic, bay leaf and sauté for about 3 minutes or just until the onion is soft and translucent.
Add the chicken stock and bring to a boil.
Stir in the rice then cover and reduce the heat to low.
Cook the rice until tender and all the liquid has absorbed, for about 20 minutes.
Remove the bay leaf and discard it.
Once the rice is cooked, transfer to a bowl and stir in the cheese.
Let the rice cool and then stir in the beaten egg and egg yolk.
Divide the mixture in 12 equal portions and place the portions on a small tray. Having these portions ready will make it easier to fill the rice balls.
Keep a small bowl filled with water close by and from time to time moisten your hands for easy rolling of the rice balls. This step will greatly minimize any fuss and mess when rolling the balls. Trust me and you can thank me later!
Combine the prosciutto and mozzarella together in a small bowl.
With moistened hands, scoop up 1/4 cup of the rice mixture and cup it in one hand. Use your other hand to flatten it slightly while making an indentation. Place about a teaspoon of the mozzarella and prosciutto mixture in the middle. Close your hand, making a fist and gently roll into a ball. If needed, you can always add more rice.
Place the rice ball on a greased cookie sheet and repeat with the remaining rice and filling to form 12 rice balls.
Moisten your hands with water as often as needed for easy rolling. This will prevent the rice from sticking too much on your hands and you will be able to roll the rice balls effortlessly.
Roll the rice balls, one at a time, first in the flour then in the egg whites and finely in the bread crumbs while turning to coat all over and place them back on a cookie sheet.
Cover with plastic wrap and then chill for 30 minutes.
In the meantime, line a cookie sheet with paper towels and place it next to your stove.
Heat the vegetable oil in a dutch oven or a deep frying pan (the oil should measure 3 inches deep), to 350*F. Use a deep frying thermometer for accuracy. Make sure there is enough oil in the pan to ensure they cook evenly and to avoid any from bursting.
Use a wire skimmer or a large slotted spoon to gently lower the rice balls into the hot oil. Do not overcrowd them.
Fry the rice balls for about 2 minutes or until golden brown and crispy all over. When ready, remove with the wire skimmer or your slotted spoon and place them on the prepared cookie sheet to drain. You can also keep them warm in the oven preheated to 200*F while you fry up the rest.
Repeat with the remaining rice balls.