Instructions

Adjust oven rack to middle position and heat oven to 400 degrees. Wrap 1 sweet potato in aluminum foil and bake until very soft and can easily be pierced with knife, 45 minutes to 1 hour. Unwrap foil and let sweet potato cool until easy to handle. Cut open potato, scoop flesh into bowl, and mash with fork until smooth; discard skin. Measure ¾ cup (175 grams) and transfer to large bowl.
Add ¾ cup ricotta; ⅓ cup (2 ounces) potato starch; 4 ounces frozen spinach, thawed, squeezed dry, and chopped; 1 cup grated Parmesan; 1 large egg; ½ teaspoon table salt; and ¼ teaspoon ground nutmeg and stir until well combined. (Mixture should be moist but firm enough to hold together when scooped.)
Line rimmed baking sheet with parchment paper. Add 1 cup corn flour to shallow bowl. Lightly dust sheet and hands with corn flour. Portion out 1½ tablespoons dough and, using both hands, roll into smooth ball, then shape into oval. Drop in bowl with corn flour and gently roll to coat. Transfer to prepared baking sheet. Repeat with remaining dough to make about 24 gnudi. Refrigerate at least 2 hours or up to 24 hours (cover if chilling longer than 2 hours).
Combine 4 quarts water and 1 tablespoon table salt in large pot and bring to boil over high heat. Once boiling, melt 3 tablespoons salted butter in 12-inch skillet over medium-high heat. Cook, stirring, until milk solids just start to turn golden, 2 to 4 minutes. Remove skillet from heat. Add half of gnudi to boiling water and cook for 1 minute. Using slotted spoon, transfer gnudi directly to skillet with lightly browned butter and return skillet to medium-high heat. Cook gnudi, undisturbed, until browned on first side, about 1 minute. Flip and cook until second side is browned, about 1 minute longer. Transfer to serving platter.
Repeat with remaining 3 tablespoons butter and gnudi. Serve immediately with extra Parmesan, lemon wedges, and flake sea salt.