Make sure your puff pastry sheet is thawed, then place it on a clean surface that's been lightly dusted with flour.
With a floured rolling pin, roll out the puff pastry, rolling from the center outward, just to reduce the thickness of the puff pastry sheet by a bit. Don't roll it too thin - and work quickly. Warm puff pastry is evil to work with.
Mix the melted butter with the Dijon mustard, parsley, and garlic powder.
Brush the butter-Dijon sauce over the surface of the puff pastry, ensuring you go right to the edges.
Sprinkle shredded Parmesan over the top.
Add the prosciutto in one layer over the top of the puff pastry, making sure to go from edge to edge. The slices can overlap, but don't layer them.
Working on the long side of the puff pastry sheet closest to you, begin to roll the dough. Try to keep it in a fairly tight cylinder and keep it even as you roll it.
Pop the rolled puff pastry into the fridge for 20-30 minutes to firm up.
Slice the puff pastry roll into discs, each about 1/3 inch thick. Place them on a sheet pan layered with parchment paper and bake at 400-F for about 20 minutes or until the puff pastry is puffed and golden brown.
Let cool and enjoy!