Preheat oven to 375 F. Grease 2, 12-cup muffin pans. . In a small bowl, stir yeast with 1 teaspoon sugar into warm water; let sit until it begins to foam, about 5 minutes. In small saucepan, slowly heat milk and butter together until butter is melted. Pour into large mixing bowl and let cool to lukewarm. Add remaining sugar, sweet potato puree, salt, lemon zest, and nutmeg to milk into mixing bowl; stir to combine. Stir in yeast mixture. Add 2 cups of flour and beat for several minutes until dough becomes elastic. Continue adding flour, about 1/2 cup at a time, until dough comes together in a ball and is firm enough to knead. Turn out onto lightly floured surface and knead, adding flour as needed to prevent sticking. Work dough until smooth and elastic, about 10 minutes. Pour the melted butter into a bowl, add dough turning several times to coat with butter. Cover and let rise in warm place until double in size, about 1 hour. Punch dough down and cut into 24 equal pieces. Divide each piece of dough into 3 smaller pieces. Roll to shape into round balls with the palm of your hand against the work surface. Put 3 balls into 1 muffin cup. Continue with remaining 23 pieces. Before baking, brush each roll generously with melted butter. Cover and let rise in warm place until almost double, about 30-45 minutes. Bake for 20-25 minutes until light brown. Makes 24 rolls.