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 (16x30 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.