Step 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.
Step 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.
Step 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.
Step 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.
Step 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.
Step 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.
Step 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.
Step 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.
Step 9
Preheat the oven to 350 degrees F (175 degrees C).
Step 9
Mix egg and water together and brush mixture on top of the panettone. Cut 2 slits crosswise on top.