Grease four ramekins. Line the bottom of each with a round of parchment
First prepare the pudding batter. Stir together the baking soda with the boiling water. Let cool.
Meanwhile, cream the butter and the granulated sugar. Then add the egg. I used a hand blender for these steps.
Stir in the flour, 1 tablespoon marmalade, and the dissolved baking soda mixture.
Place 1 teaspoon of marmalade at the bottom of each prepared ramekin, then divide the pudding batter evenly among the ramekins.
Cover each ramekin with a large square of greased parchment and a large square of foil. Wrap tightly and secure with twine.
Place the four ramekins in a large pot lined with a metal rack (or something to elevate off the bottom of the pan—I used a bunch of jar lids)
Fill pot with boiling water (hot water is okay too) until it reaches half way up the ramekins. Cover the pot with a lid. Steam the puddings for one hour on the stove (check that there is sufficient water every 20 minutes or so).
Meanwhile, prepare the quick caramel sauce by whisking together the yolk, brown sugar, water, and butter in a small saucepan. Heat on medium heat until the mixture comes to a boil, whisking constantly. Let the mixture boil for a full minute, then immediately strain it into a bowl.
Stir in the vanilla.
When the puddings are steamed. Remove the ramekins from the water bath using tongs. Let sit for about 5–10 minutes, then unwrap and run a knife along the edge to loosen the cakes. Invert the puddings on serving plates. Drizzle the quick caramel generously over each of the puddings.