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1
The day before baking, mix flour, cold water, and sourdough starter together in a bowl. Cover and leave out at room temperature, 8 hours to overnight.
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2
At the same time, combine raisins, cherries, and pineapple in a separate bowl. Stir in rum. Let fruit soak, tossing occasionally, 8 hours to overnight.
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3
Pour warm water into large bowl of a stand mixer fitted with a dough hook attachment. Add yeast and let dissolve for 10 minutes. Add eggs, 1/3 cup plus 2 tablespoons sugar, vanilla extract, orange zest, and lemon zest. Whisk together. Scoop in the panettone starter. Add flour and salt.
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4
Knead until dough is very smooth and elastic, about 10 minutes. Stop and scrape down the sides if needed. Add butter; knead until butter is completely mixed in and dough is soft and somewhat sticky, about 5 minutes. Transfer to your work surface.
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5
Toss and fold your dough into a rough ball shape using wet fingers and a bench scraper. Transfer dough back to the bowl. Cover and let rise until doubled, about 3 hours.
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6
Transfer dough back to your work surface and shape into a rough ball. Place into a resealable plastic bag. Refrigerate, 8 hours to overnight.
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7
Remove dough from the bag and press out into a flat rectangle. Sprinkle with flour and roll out to at least 1/2-inch thickness. Spread the dried fruit all over the surface. Roll dough up into a log, sealing in the fruit. Roll both ends of the log toward the middle and smooth it over into a ball.
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8
Place dough ball into a short, wide paper panettone mold. Cover with plastic wrap and let rise until dough is at least 2/3 of the way up the sides, 3 to 4 hours.
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9
Preheat the oven to 350 degrees F (175 degrees C).
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10
Mix egg and water together and brush mixture on top of the panettone. Cut 2 slits crosswise on top.
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11
Bake in the preheated oven until beautifully browned, 40 to 45 minutes. Poke 2 skewers through either side of the panettone and flip it over into a panettone hole or a Dutch oven. Let cool upside-down for 2 hours.
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12
Pull out the skewers and slice into pieces.
