Sweet Potato Gnocchi

Bahan-bahan (2 porsi)

• 2 medium sweet potatoes (about 1 1/4 pounds)
• 1 large egg yolk
• 1/2 cup grated Parmesan cheese (plus additional for serving)
• 2/3 cup all-purpose flour (plus additional for your counter)
• ½ teaspoon kosher salt
• 4 tablespoons unsalted butter
• 1 tablespoon minced fresh sage
• 1/4 teaspoon kosher salt
• 1/4 teaspoon apple cider vinegar