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 1/2" 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 scraps separately, until golden brown, for a baker’s snack, if desired.) , 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. , Storage information: Store any leftover giant chocolate croissant, covered, at room temperature for up to 4 days.