Place the flour, sugar, yeast, salt, yolks, zests, vanilla, cinnamon & ginger into the bowl of a stand mixer. Next, warm the milk up to 38°c/100°f (in a microwave or on the stove) & add into the flour.
Using the dough hook, mix the dough on a medium speed until it has come away from the sides of the bowl & is looking smoother. This will take 3-4 minutes.
With the mixer still running on medium, add in the butter a small amount at a time, incorporating each piece fully before adding more. Once all the butter has been incorporated, keep mixing until the dough passes the window pane test, which will take roughly 5-10 minutes.
Next, add the soaked fruit to the mixing bowl & incorporate into the dough with your hands. Transfer the dough to a lightly oiled bowl & leave to prove at room temperature until double in size. This will take between 1-1½ hours.
When the dough has almost finished proving, cut the marzipan into 5 x 20g pieces. Roll each into a ball then flatten into 1 cm thick discs. At the same time, line a large baking tray with baking paper & set aside. Once doubled in size, knock the dough back with your hands then transfer to a lightly floured worksurface.
Cut the dough into 5 equal portions, each weighing around 135g.
Next, flatten a piece of dough into a 1½cm thick disc then place a piece of marzipan in the middle. Fold the sides of dough up & over the marzipan (so that it is completely covered) then flip the dough upside down & use your hands to shape into a ball. Place seam side down on the lined baking tray & repeat with the remaining dough. Once shaped, cover the tray with another baking tray (flipped upside down) or loosely with oiled clingfilm & leave to rise at room temperature for 1-1½ hours. Once proved the buns will be well risen but not quite doubled in size. Whilst the buns are proving, preheat an oven to 180°c/160°c fan (350°f/320°f).
In a bowl, whisk together the egg, milk & salt for the egg wash then brush over the top of each bun.
Bake the stollen in the preheated oven for 15-20 minutes, until a deep golden brown then leave to cool on the tray for 10 minutes (or until cool enough to handle but still warm).