1. Place the warm water in a small bowl, and sprinkle with yeast. Let stand until yeast is dissolved and mixture is foamy, about 7 minutes.

2. In a small saucepan, heat milk over medium heat just until it begins to steam and bubble around the sides. Remove from heat; add the butter, and stir until melted and combined. Stir in sugar, salt, and cardamom. Let cool slightly.

3. Preheat oven to 400 degrees. Line a baking sheet with parchment paper; set aside. Combine sweet potatoes and lemon juice in the bowl of an electric mixer fitted with the paddle attachment; beat until smooth, 2 to 3 minutes. Beat in egg and milk and yeast mixtures until smooth.

4. Switch to the dough-hook attachment. Add flour, 1 cup at a time, beating until a stiff dough forms. Continue kneading dough on medium speed until smooth, about 8 minutes. The dough will still be slightly sticky.

5. Transfer to a large oiled bowl. Cover with a clean kitchen towel, and let stand in a warm place to rise until dough is doubled in bulk, about 1 hour.

6. Punch down dough, and knead again with your hands, just until smooth. Using a bench scraper or sharp knife, cut dough into 20 equal pieces, and shape into rolls.

7. Place rolls on prepared baking sheet, about 2 inches apart; cover with a kitchen towel, and let rise again in a warm place until doubled in bulk, about 40 minutes.

8. Using kitchen scissors or a sharp paring knife, snip an X in the top of each roll. Brush rolls with melted butter. Bake until tops of rolls are golden, about 20 minutes, rotating pan halfway through. Transfer to a wire rack; serve hot or at room temperature.