Remove two sheets of puff pastry from the freezer and allow to come to room temperature. Dock both liberally with a dough docker then place each one on a half sheet tray lined with parchment paper. Place another sheet of parchment on top of the dough and then another half sheet tray so that the dough is sandwiched in. Bake at 375°F until browned all the way through. Remove pastry from oven and let cool completely with the sheet on top.
Once cool, remove the top sheet. Place each pastry sheet on separate cake boards. In a mixer fitted with the whip attachment, mix together 1 pound speedy crème mix and 1 pound 8 oz cold water. Whip until no lumps remain then remove from mixer and immediately spread completely to the edges of one pastry sheet. Reserve.
Warm up both chocolate and white flat icings. On the second pastry slab, spread chocolate flat icing to the edges, working quickly, pipe stripes of white flat icing on top. Drag a paring knife through the stripes, changing direction each time. Place in cooler to set up. Once set, place chocolate iced pastry sheet onto the sheet covered with custard. Trim edges and cut into desired serving size.