To make the chocolate filling: In a small bowl, stir together the sugar, cocoa, flour, and salt. Add the melted butter, water, and vanilla and stir until smooth. Set aside while you roll out the puff pastry.

To assemble the chocolate croissant: Divide the puff pastry in half (about 290g per piece). Return one half, wrapped, to the refrigerator until ready to use.

Dust a clean work surface with flour. Working with the other half of the puff pastry, roll it into a 12" x 16" rectangle (the pastry will be quite thin). Transfer the rectangle to a parchment-lined baking sheet and spread the chocolate filling in an even layer over the surface, leaving ½" bare on every side. Sprinkle the mini chocolate chips or chopped chocolate evenly over the filling and set aside.

Roll the remaining half of puff pastry to another 12" x 16" rectangle. Brush the bare edges of the filling-covered puff pastry with egg wash, then place the second rectangle on top. Firmly press the edges to seal, then refrigerate the assembled chocolate croissant for 30 minutes, uncovered.

While the assembled chocolate croissant is chilling, preheat the oven to 400°F with a rack in the center.

Remove the pan from the refrigerator and trim the edges of the giant chocolate croissant with a sharp chef’s knife or pizza wheel; this will help create clean edges that will rise high when baking.

Bake the giant chocolate croissant for 25 to 30 minutes, until browned.

Remove it from the oven and cool directly on the baking sheet. Generously dust with non-melting or confectioners’ sugar before cutting and serving warm or at room temperature.