Step 1

Dissolve the yeast in the warm milk or water with 1 teaspoon of sugar and leave for 10 minutes, until it froths.

Step 2

Beat the rest of the sugar with the egg and the yolk. Add the sour cream or oil, the salt, vanilla, and yeast mixture, and beat very well. Fold in the flour gradually, and continue beating until you have a soft, smooth, and elastic dough, adding more flour if necessary. Then knead for 5 minutes, sprinkling with a little flour if it is too sticky. Coat the dough with oil by pouring a drop in the bowl and turning the dough in it. Cover the bowl with plastic wrap and leave in a warm place to rise for about 2 hours, or until doubled in bulk.

Step 3

Knead the dough again for a few minutes, then roll out on a floured surface with a floured rolling pin to 1/4-inch (1/2-cm) thickness. With a pastry cutter, cut into 2-inch (5-cm) rounds. Make a ball out of the scraps so as not to waste them, roll out, and cut into rounds. Put a teaspoon of jam in the center of a round of dough, brush the rim with a little water to make it sticky, and cover with another round. Press the edges together to seal. Continue with the rest of the rounds and arrange them on a floured tray. Leave them to rise for about 30 minutes.

Step 4

Heat 1-1/2 inches of oil in a saucepan to medium hot. Drop in the doughnuts, a few at a time. Fry in medium-hot oil for 3-4 minutes with the lid on until brown, then turn and fry the other side for 1 minute more. Drain on paper towels. Serve sprinkled with confectioners' sugar. They are at their best when still warm and fresh.