Beat the marzipan, sugar and butter in a food processor or stand mixer until smooth. Set aside.
Roll out the dough on a lightly floured surface to a large rectangle, about 40 x 25 cm and 3 mm thick. With the short sides of the rectangle nearest to you, spread the marzipan butter in the centre third, lengthways. Fold one-third over the filling, then brush with water. Fold the final third