Mini Beef Wellingtons

Name: Mini Beef Wellingtons
Ingredients: 2 tablespoons unsalted butter; 1 pound button mushrooms, finely chopped; 1 teaspoon fresh thyme leaves (from about 2 sprigs); 1 tablespoon sherry vinegar; 1 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; Two 8-ounce beef filets, each cut into 8 cubes (16 cubes total); 1 tablespoon olive oil; 2 tablespoons Dijon mustard; 8 thin slices prosciutto, halved crosswise; 1 sheet frozen puff pastry, thawed; 1 large egg yolk, lightly beaten; Sea salt, for sprinkling