Combine the chopped dates and baking soda in a heatproof bowl. Pour the boiling water over them, stir, and let sit for 10 minutes.

In a large bowl, beat the butter and brown sugar until pale and fluffy. Add the egg and vanilla, then beat until smooth.

Stir the date mixture into the butter mixture, then sift the flour over the top. Fold until no streaks remain.

Preheat oven to 350°F (180°C) and grease an 8-inch round cake pan. Pour the batter in, smooth the top, and bake for 25 to 30 minutes, until a skewer comes out with moist crumbs.

While the pudding bakes, combine the sauce ingredients in a small saucepan. Stir over medium heat until the butter melts, then let bubble for 2 minutes. Remove from heat.

Poke the hot pudding all over with a skewer, then pour half the sauce over the top. Let it sit for 5 minutes, then serve warm with the rest of the sauce.