Preheat oven to 375 degrees F. Coat four 10-ounce ramekins (or a 2- to 2 1/2-quart souffle dish) with butter and place them on a baking sheet.
Heat the olive oil in a small frying pan then add the shallots, garlic, Italian seasoning, and mushrooms. Sauté until cooked through 5-6 minutes.
In a separate saucepan over medium-high heat, melt the unsalted butter and truffle butter. Whisk in the flour and cook, whisking, for 1 minute. Adjust heat as needed to prevent the mixture from getting too dark; it should be the color of caramel. Add in the milk, mustard, and truffle salt and cook, whisking constantly, until thickened, 1 to 2 minutes. Remove from heat and immediately whisk in goat cheese and 3 egg yolks until well combined. Transfer to a large bowl.
Beat the 5 egg whites in a medium bowl with an electric mixer on high speed until soft peaks form. Add the cream of tartar and continue beating until stiff peaks form.
Gently fold half of the whipped whites into the milk-egg mixture. Gently fold in the remaining egg whites and the reserved mushrooms just until no white streaks remain. Transfer to the prepared ramekins or souffle dish.
Bake until puffed, firm to the touch, and an instant-read thermometer inserted into the center registers 160 degrees F, about 20 minutes in ramekins or 30 minutes in a souffle dish. Serve immediately.