Melt butter in a large pot over medium heat. Add shallots, season with salt and pepper, and cook for about 3 minutes or until soft and translucent. Add rice and stir to coat in butter. Cook, stirring often, until edges of rice grains are translucent, 3- 5 minutes.

Stir in mushrooms and wine, season with salt and black pepper. Bring to a simmer and cook, stirring constantly, until liquid is fully absorbed. 

Add a ladleful of hot broth and stir until the liquid is fully absorbed. Repeat adding broth in 1 cup increments, stirring frequently and simmering gently, until rice is al dente (you may not need all of the broth) and liquid is mostly absorbed, about 25 minutes. Stir in parmesan, truffle oil and season with salt and black pepper.

Spread out risotto in an even layer on a baking sheet and let cool. Cover loosely with plastic wrap and chill at least 30 minutes until cold and firm. If making risotto ahead of time, it’s fine to store it in a covered bowl.

Line another baking sheet with parchment paper. Take 1/4 cup of cooled risotto and shape into a ball. Insert a piece of mozzarella into the center, squeeze rice closed around it, and roll until smooth (to prevent sticking, it helps to lightly wet your hands every few balls). Place on the prepared baking sheet and repeat with remaining rice and mozzarella. Freeze balls for at least 10 minutes.

Place flour, eggs and breadcrumbs in 3 separate bowls. Season them all with salt and pepper and stir to combine. To bread arancini, coat one ball in flour and tap off excess. Coat in eggs, letting excess drip off, then roll in breadcrumbs and press gently to adhere. Transfer to a parchment paper lined baking sheet and repeat breading remaining arancini. Refrigerate while you heat up oil. Arancini can be refrigerated overnight (covered loosely, save breadcrumbs to re-coat before frying) or frozen at this point.

Heat 1-inch of oil in a large, heavy-bottomed pot over medium heat to 350°F.

Fry a few arancini at time, rolling around occasionally to brown evenly, until golden and crispy, 5- 7 minutes. Drain on a paper towel lined plate, sprinkle with salt. Let oil return to 350°F before repeating with remaining arancini. Serve while hot.