1. Heat milk in small saucepan over low heat until just warm, about 110 degrees F. Pour into a small bowl; whisk in the yeast. Stir in the flour until smooth. Cover the bowl tightly with plastic wrap; let the sponge rise at room temperature for about 30 minutes.Dough:
2. Beat the butter in large bowl until soft and light. Beat in salt, sugar, lemon zest, vanilla and rum; continue beating until light and smooth, about 5 minutes.
3. Combine the eggs and yolks in a small bowl. Beat a third into the butter mixture; continue to beat until the mixture is smooth. At the lowest speed, add a third of the flour and beat until it is absorbed. Scrape down the side of the bowl and the beaters. Add another third of the egg mixture as above, then another third of the flour as above. Then add the remaining egg mixture, beating until smooth. Scrape down the side of the the bowl. Beat in the remaining flour until it is absorbed. Scrape in the sponge; beat at lowest speed for about 5 minutes or until the dough is smooth and slightly elastic. Beat in the candied peel and raisins until evenly distributed.
4. Butter a large bowl; add the dough and turn it to thoroughly coat. Cover the bowl tightly with plastic wrap and let the dough rise at room temperature for up to 1-12 hours or until doubled in volume.
5. Butter and flour a 9 x 3-inch springform pan. Stir dough with a rubber spatula to deflate it, then scrape into the prepared pan. Butter a piece of plastic wrap and drape loosely over the pan, buttered side down. Let the dough rise again at room temperature for about 1 hour or until it reaches the top of the pan.
6. When the dough has almost reached the top of the pan, position an oven rack in the middle level of the oven. Heat oven to 375 degree F.
7. When the dough has risen completely, uncover and place in the oven. Bake in 375 degree F oven for about 20 minutes or until the dough is well risen and deeply colored. Then cover the top loosely with a piece of foil. Bake for 25 to 30 minutes longer or until a thin knife inserted in the center emerges clean.
8. Cool in the pan on a rack for 5 minutes. Remove side of pan. Slide panettone off the base to a rack. Paint with the melted butter and let cool. Makes one 9-inch round bread.