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1
Dissolve the yeast in the warm milk with a pinch of the sugar and leave for 5–10 minutes until the surface shows a light foam, confirming the yeast is active. The pinch of sugar gives the yeast something to feed on immediately, so the foam appears quickly and you can be certain the yeast has life before it enters the dough.
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2
Combine the flour, remaining sugar, salt, and lemon zest in a large bowl, then add the eggs, butter, lard, and the yeast-milk mixture. Work by hand or in a stand mixer for 10 minutes until the dough is smooth, soft, and slightly tacky — it should pull cleanly from the sides of the bowl but still yield gently to a pressed finger. The full 10 minutes of working builds the dough's structure so it can hold the gas produced during rising without tearing or going slack.
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3
Cover the dough and leave to rise at room temperature for 1 hour, by which point it should have doubled visibly in volume and feel airy when pressed. Keeping it at room temperature rather than somewhere warm lets the rise happen slowly enough that the dough develops flavour as well as volume.
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4
Divide the dough into pieces of approximately 60 g each and roll each into a smooth ball, then rest uncovered for 10 minutes so the dough relaxes and stops springing back before you roll it. Roll each ball to a thickness of 1 cm and cut into rounds of 8 cm diameter, arranging them in two equal sets.
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5
Place a small, level teaspoon of jam at the centre of one set of rounds, then lay the remaining rounds directly over each filled base. Press the edges firmly all the way around with the fingertips to produce a tight seal with no jam visible at the join, because a weak seal will allow jam to escape into the hot oil, collapsing both the doughnut and the fry. This is where the dish lives or dies.
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6
Transfer the sealed krapfen to a lightly floured tray, cover with a cloth, and leave to prove for 30–45 minutes. The rounds should puff noticeably and look visibly lighter, which tells you the dough is aerated enough to open up properly in the oil rather than fry dense.
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7
Heat the lard or oil in a wide, deep pan to 170°C — no higher — ensuring the fat is deep enough for the krapfen to float without being fully submerged, since this shallow float is what permits the white equator to form around the middle. Lower the krapfen gently into the oil and fry for 2–3 minutes per side without pressing or rushing; the sides should be deep gold, and a pale, un-fried white band should encircle the middle where the dough floats clear of the oil, confirming that the temperature was correct and the interior has had time to cook and puff through.
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8
Remove with a slotted spoon and drain briefly on a rack or absorbent paper. While still warm, dust generously with icing sugar so it dissolves slightly against the warm dough on contact.