Two nights before baking, prepare the levain by mixing together the sourdough starter, 125g of the flour, and 60g of milk. The resulting levain will be stiff. Cover and leave to ferment overnight.
The next morning, the make the dough. Mix together the levain and milk, followed by the flour, sugar, and salt.
Knead in the butter, and continue kneading until the dough comes together (about 15 minutes) and is somewhat smooth in appearance. The goal is to start building up the gluten early so that the fillings can be incorporated without the dough coming apart.
Cover the dough and allow to ferment for 1 hour.
On floured surface, stretch the dough into rectangle and sprinkle ⅓ of the fruit, peel, and almonds. Gently roll up the dough. Repeat this process two more times to incorporate all of the fillings. Then form the dough into a round and cover.
Let ferment for another 3-5 hours, depending on the temperature of the dough. The dough should form a domed top and have almost doubled in size.
Stretch the dough out into a rectangle roughly the desired length of the loaf. Roll the marzipan until it is just slightly shorter than the length of the dough and place the log in the center. Roll the dough up around the marzipan and place it down the seam is on the bottom, then tuck the ends underneath and ensure all of the marzipan is sealed within the dough.
Cover and put the dough in the refridgerator to proof overnight.
2 hours before baking, remove the dough from the refridgerator to temper.
Preheat the oven to 350°F and bake for 35-40 minutes, until lightly golden and cooked through.
As soon as the bread is baked, brush with melted butter and sift powdered sugar all over it.