Your starter should have doubled in size. Place it in a large mixing bowl, add flour, sugar, milk and a pinch of salt. Mix together into a cohesive dough. It will be very firm, but don't worry. It will change once we add butter and fruit.
Cut room temperature butter into cubes. One piece at a time, slowly work the butter into the dough, kneeding it until butter is fully incorporated. Then pop the dough back into the bowl. Cover and leave to rest for an hour.
In the meantime, finely chop blanched almonds and weight out the mixed peel. Drain the dried fruit and squeeze out excess brandy.
Get the dough out onto a lightly floured surface. Stretch it out into a rough rectangle and sprinkle 1/4 of your dried fruit, 1/4 of the peel and 1/4 of almonds onto it. roll it up and flatten again into a rectangle. Sprinkle with fruit, peel and nuts again. Roll it again, but in the opposite direction to last time. Repeat twice more. This will make sure the fruit is evenly distributed in your dough.
Knead your dough for 10 minutes until it's smooth, soft and elastic. Then shape into a tight ball and place into a large clean bowl. Cover and leave at 21-25°C temperature to rise for 2-3 hours, or until doubled in size. If your kitchen is much colder than that, see tips in the text above.
Once doubled, scoop the dough out of the bowl onto a floured surface and roll out into 30x20 cm rectangle.
In a different spot, dust your work surface with icing sugar and roll out your marzipan into a thin 20x12cm rectangle. Make sure your marzipan sheet is smaller than your dough sheet.
Brush the surface of the dough with some melted butter and place the marzipan on top in the centre of the dough rectangle. From the shorter end of the rectangle, tighly roll the stollen, leaving the seam at the bottom.
Tuck both edges of the roll under to fully encase the marzipan so that it doesn't leak out during baking.
Transfer the Stollen roll onto a baking parchment-lined sheet. Carefully cover with lightly-greased clingfilm or a clean plastic bag and leave to rise again for 2-3 hours until just about doubled in size.
Once doubled, preheat the oven to 170°C Fan. Place your Sourdough Stollen in the centre of the oven and bake for 45-50 minutes.
When cooked through, take it out of the oven and immediately brush with melted butter. Sift 2 tbsps of icing sugar all over the top and sides and leave to cool completely before slicing.