Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Prepare the mocha filling: In a small bowl, mix espresso and melted chocolate until smooth. Let it cool slightly.
Roll the pastry: Unfold the puff pastry and cut into triangles. Spoon a little mocha filling onto the wide end of each triangle.
Roll the croissants: Starting from the wide end, roll up each triangle tightly into a croissant shape.
Egg wash: Brush the tops of each croissant with the beaten egg for a golden finish.
Bake: Place on the baking sheet and bake for 15-20 minutes, until golden and flaky.
Make the glaze: In a small bowl, whisk together powdered sugar, cocoa powder, espresso, and vanilla. Drizzle over baked croissants.
Serve: Let the glaze set for a minute, then enjoy your mocha croissants with a cup of coffee.