Pre-heat the oven to 180C and line a baking sheet with aluminium foil and baking paper.
Place the chicken fillet on a chopping board and bash it out until it is about 2cm thick.
Lay one sheet of pastry out, brush it with butter then place another sheet on top then brush it with butter too.
Lay out one chicken fillet then pile the middle with cheese, spinach and folded proscuitto (see image).
Fold the left side of the chicken over the filling then the right side and then fold the left side of the pastry over the top.
Brush it with butter and fold the top of the pastry down over the chicken, brush it with butter and fold up the bottom.
Finally, brush the rest of the pastry with butter and roll up the chicken.
Place it on a baking dish seam-side down and brush with butter.
At this stage you can freeze the parcels. Freeze in a single layer. Defrost before cooking.
Bake for 25 minutes or until browned.