Preparation

• Generously butter the inside of a 1-quart, high-sided soufflé dish; also lightly butter one side of a 12-inch piece of aluminum foil.
• Mix the flour, baking powder, and salt in a small bowl. Set aside.
• In a large bowl, combine the butter and sugar with an electric mixer at medium speed, scraping down the inside of the bowl occasionally with a rubber spatula, until light and fluffy. Beat in the marmalade, then the eggs one at a time, and finally the vanilla.
• Stir in the dry ingredients and walnuts by hand, just until there are no pockets of unmoistened flour. Scrape the batter into the soufflé dish.
• Set the foil butter side down over the soufflé dish and seal tightly. Bring 5 cups water to a boil on the stove. Meanwhile, cut a piece of cheesecloth large enough to hold the soufflé dish (16×30 inches). Set the dish in the center and tie each set of opposite ends together in a knot over the dish. Lifting the dish by holding on to one of the knots, set it in the slow cooker. Pour in enough boiling water to come about two-thirds of the way up the side of the dish. Cover the slow cooker and cook on high until a toothpick inserted through the cheesecloth and foil into the center of the cake comes out clean, 3 to 3-1/2 hours.
• Holding the knotted cheesecloth with tongs or a waterproof mitt, transfer the soufflé dish to a wire rack. Cut off the cheesecloth, remove the foil, and cool for 10 minutes. Unmold onto a serving plate and let cool for another 10 minutes. Serve sliced into wedges.