For the carnival doughnuts, crumble the yeast into a large enough bowl, stir with the milk and 1 tsp. sugar, and let rise for about 15 minutes with the lid closed.
Stir half of the flour with the remaining sugar, salt, lemon zest and eggs into the dampfl. Knead in the butter and the rest of the flour well and beat the dough until it comes away from the side of the bowl.
Let the yeast dough rise for 45 minutes with the lid closed. Knead the dough well repeatedly and roll out on a floured surface to a thickness of 1 cm.
Using an 8 cm diameter cup, cut out 24 discs. Place 1 tsp. jam in the center of each of 12 slices, brush the edges with water, place an empty dough slice on top and press the edge firmly.
Let the doughnuts rise on a floured surface covered with a tea towel for another half hour.
Heat the fat to 175 degrees. Bake the doughnuts in batches for 4-5 minutes on each side until golden brown, drain and sprinkle with powdered sugar.