Season chicken breasts with salt and pepper. Heat olive oil in a skillet over medium-high heat, and sear chicken for 4-5 minutes on each side. Remove and cool.

In the same skillet, melt butter, add mushrooms, shallots, and garlic. Cook for 8-10 minutes until thickened into a paste. Cool completely.

Lay prosciutto on plastic wrap, spread cooled mushroom duxelles on top, place chicken in center, roll tightly, and chill for 15-20 minutes.

Roll out puff pastry into a rectangle, place chicken roll in center, fold pastry over, seal edges, and refrigerate for 10 minutes.

Preheat oven to 400°F (200°C). Brush pastry with egg yolk mixture, cut slits on top, and bake for 25-30 minutes until golden and internal temp reaches 165°F (74°C). Rest for 5 minutes before serving.