Place the flour into the bowl of a food mixer fitted with a dough hook. Add the salt and sugar to one side and the yeast to the other. Add the milk and eggs and mix on a slow speed for two minutes. Increase the speed and mix for a further 6-8 minutes. The dough should be glossy and elastic.
Add the softened butter and continue mixing for five minutes, scraping down the bowl periodically to ensure that the butter is thoroughly incorporated. The dough will be very soft at this stage.
Place the dough into a bowl, cover and chill overnight to allow the dough to firm up.
When you’re ready to assemble the pie, preheat the oven to 200C/400F/Gas 6.
Tip the dough onto a lightly floured surface and fold it in on itself a few times to knock out the air. Roll the dough out to 5mm/¼in thickness and into a large circle. Lay half the prosciutto on the base. Spread the cranberry sauce over the top. Place the brie on top of the cranberry sauce. Use the remaining prosciutto to cover the brie.
Stretch the dough over the ham to encase the brie and seal the edges.
Turn over your brie parcel and place on a baking tray. Glaze the dough with the beaten egg. Using the back of a knife score a criss-cross pattern over the parcel.
Bake for 20-25 minutes, or until golden-brown.