Directions

1. In a large bowl, stir together the warm water and yeast. Let stand until foamy, about 5 minutes. Add 3/4 cup flour, 1/4 cup sugar, and salt; mix until well combined. Add egg yolks and remaining 1 3/4 cups flour. Mix until combined, then knead dough in bowl until all flour is incorporated. Turn out onto a lightly floured work surface; knead a few minutes until smooth. Knead in margarine until incorporated.
2. Transfer dough to a well-oiled bowl, turning several times to coat entirely with oil. Cover tightly with plastic wrap, and refrigerate overnight.
3. Bring dough to room temperature, about 30 minutes. On a lightly floured work surface, roll out dough into an 11-inch square, about 1/4 inch thick. Cut out about 24 (2-inch) rounds. Reroll scraps; cut out about 16 more rounds.
4. Line a baking sheet with a clean kitchen towel. In a small bowl, lightly beat egg whites. Brush edge of a dough round with egg white, then mound 1/2 teaspoon jam in center. Top with another round; press edges to seal. Repeat with remaining rounds. Transfer to prepared baking sheet; let rise until puffy, 20 to 30 minutes.
5. Heat 3 inches of oil in a large, heavy pot until it reaches 360 on a deep-fry thermometer. Working in batches of 4 or 5, slip doughnuts into hot oil. Fry, turning once, until golden brown, about 1 minute. Using a slotted spoon, transfer doughnuts to paper towels to drain.
6. While doughnuts are still hot, dip them in remaining 1/2 cup sugar, turning to coat. Serve immediately.