Mix the flour, 1 tsp salt, caster sugar, yeast, milk and eggs together in a mixer using the dough attachment for 5 mins until the dough is smooth.
Add the butter and mix for a further 4 mins on medium speed.
Scrape the dough bowl and mix again for 1 min.
Place the dough in a container, cover with cling film and leave in the fridge for at least 6 hrs before using.
Wrap the Brie in the prosciutto and set aside.
Turn out the dough onto a lightly floured surface.
Roll into a 25cm circle.
Place the wrapped Brie in the middle of the circle and fold the edges in neatly.
Put the parcel onto a baking tray lined with baking parchment and brush with beaten egg.
Chill in the fridge for 30 mins, then brush again with beaten egg and chill for a further 30 mins.
Leave to rise for 1 hr at room temperature.
Heat oven to 200C/180C fan/gas 6, then bake for 22 mins.