Sprinkle or crumble yeast into a large bowl. Add 1/4 cup of lukewarm milk. Use very warm milk for active dry yeast and lukwarm for compressed yeast. After a minute or two, stir to dissolve.
Add melted butter, sugar, eggs (one at a time, beating after each addition), lemon rind and salt. Gradually add enough flour to make a soft dough; beat well after each addition. Form into a ball and put in a greased bowl.
Turn over so top of dough is greased. Place in a warm place and let rise, covered, until doubled in bulk. Turn out on a floured board and knead well. Roll out to a 1/4-inch thickness; cut into rounds with a 3-inch cookie cutter or glass. Place a small amount of plum or apricot preserves in center of half the rounds.
Moisten edges with egg white or water; top with other rounds of dough. Press firmly together to completely enclose preserves. Arrange on a large cookie sheet; put in a warm place, covered, until large and light. Drop into a kettle of hot deep fat (370C° on a frying thermometer) and cook until golden brown on both sides, only a few mins. Revoce with a slotted spoon and drain. Serve warm, sprinkled with confectioners' sugar. Makes about 30.
Notes: These well-known fruit-filled yeast doughnuts, a famous specialty of Berlin, are essential for the convivial celebration of the New Year.