Fill a large heavy pot (at least 10 inches across, with a tight-fitting lid) with 1‚ inches water. Make a platform for pudding by setting metal cookie cutters or egg-poaching rings in bottom of pot.
Center a 10- by 8-inch sheet of wax paper or parchment paper on top of a 14-inch square of heavy-duty foil with a short edge toward you.
Roll out dough on a lightly floured surface to form a 10- by 6-inch rectangle. Spread jam over dough, leaving a ‚-inch border around edges, and brush border with water. Starting with a short side, roll up dough to form a 6-inch log. Gently pinch seam to seal.
Center log, seam side down, on wax paper and brush with melted butter. Fold short sides of wax paper loosely over log and wrap log loosely in foil, crimping edges together over top to seal completely and leaving space in package for pudding to expand slightly.
Bring water in pot to a boil and set mold on platform. Steam pudding, covered, over simmering water 1 1/2 to 2 hours (add more boiling water to pot if necessary), or until golden and puffed. Transfer pudding in foil to a rack and let stand 5 minutes.
Unwrap pudding and gently slide onto a platter. Slice and serve immediately with custard sauce.
