Leave the sourdough starter to rise well, wrapped in a napkin dusted with flour, until doubled in volume, in a warm dry place. The rising is a critical operation to make a good panettone. Two hours or so should be enough. Put about 1 1/4 cups of flour onto a pastry board, crumble the sourdough starter in, dissolving it with a bit of warm water, gently fold in the flour, to make a soft and smooth dough. Once made a pat of dough, place it into bowl, dusted with flour and cover with a napkin; leave to rest for 3 hours.
After said time, put onto the pastry board 2 cups of flour and place the risen dough in the centre. Add a few tablespoon of milk to the risen dough to soften it, then incorporate the flour, to obtain a smooth dough. Knead vigorously and make a ball of dough to be left to rise, as you did before, into the bowl covered with a napkin, for 2 hours. Dice the candied citron and orange, soak the sultanas in warm water for fifteen minutes and dry well. Melt 1 1/4 cups of butter, without frying. In another pan, dissolve the sugar in warm water to make a syrup, then add the whole eggs and egg yolks, cook in a bain-marie to heat up the mixture. Pour 2,2 lbs of flour onto the pastry board, add a pinch of salt and mix it to the flour. Make a well in the centre.
Place the ball of risen dough in the well, add the melted butter and mix. Now add the sugar syrup, previously blended with the eggs, and incorporate slowly, bit by bit, all the remaining flour. Knead vigorously for at least 20 minutes, amalgamating all the ingredients to make a firm smooth and elastic dough. In the end add the sultanas and candied fruit. According to the size of the oven, you can divide the dough in two or three parts. Grease your hands with butter and give a round shape to the dough; then place onto a paper sheet, buttered and floured where you will leave them to rise for six hours or so, in a warm dry place, without drafts.
The rising is complete when the dough has doubled in volume. Make an cross incision on the surface. Place the cakes in cool place for a few minutes, then into the oven at 400-425° F. After five minutes in the oven, pour the melted butter into the cross incision on the top, and while baking, carefully turn the heat down reducing the temperature to prevent from burning.
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