Preheat oven to 350 degrees F (180 degrees C) on conventional bake setting.
Unroll puff pastry onto a clean surface. Cut into 10 to 14 triangles.
Starting at the wide end of each triangle, roll the dough toward the tip to form a crescent shape. Place on a parchment-lined baking sheet with the tip tucked underneath.
Whisk egg yolk and milk together. Brush generously over each croissant.
Bake for 20 to 25 minutes until deeply golden and puffed.
Remove from oven and allow to cool completely, about 15 to 20 minutes.
Once cooled, slice each croissant in half horizontally.
Spread a layer of cream cheese on the bottom half.
Add a small amount of arugula.
Layer prosciutto on top.
Replace the top half and serve on a tray.